PMMA-silica hybrid preparation conditions.
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",isbn:"978-1-83881-111-2",printIsbn:"978-1-83880-992-8",pdfIsbn:"978-1-83881-112-9",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!0,isSalesforceBook:!1,isNomenclature:!1,hash:"acb2875b3bfc189c9881a9b44b6a5184",bookSignature:"Dr. Abdo Abou Jaoudé",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/11865.jpg",keywords:"Linear Operators, Normal Operators, Spectral Theorem, Applications, Differential Operators, Integral Operators, Functional Calculus, Complex Variables, Complex Analysis, Theory, Recent Advances, Latest Trends",numberOfDownloads:null,numberOfWosCitations:0,numberOfCrossrefCitations:null,numberOfDimensionsCitations:null,numberOfTotalCitations:null,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"April 13th 2022",dateEndSecondStepPublish:"June 21st 2022",dateEndThirdStepPublish:"August 20th 2022",dateEndFourthStepPublish:"November 8th 2022",dateEndFifthStepPublish:"January 7th 2023",dateConfirmationOfParticipation:null,remainingDaysToSecondStep:"2 months",secondStepPassed:!0,areRegistrationsClosed:!1,currentStepOfPublishingProcess:3,editedByType:null,kuFlag:!1,biosketch:"Abdo Abou Jaoudé is a pioneering Associate Professor of Mathematics and Statistics at Notre Dame University-Louaizé. He holds two PhDs in Mathematics and Prognostics from the Lebanese University and Aix-Marseille University. His research interests are in the field of mathematics.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"248271",title:"Dr.",name:"Abdo",middleName:null,surname:"Abou Jaoudé",slug:"abdo-abou-jaoude",fullName:"Abdo Abou Jaoudé",profilePictureURL:"https://mts.intechopen.com/storage/users/248271/images/system/248271.jpg",biography:"Abdo Abou Jaoudé has been teaching for many years and has a passion for researching and teaching mathematics. He is currently an Associate Professor of Mathematics and Statistics at Notre Dame University-Louaizé (NDU), Lebanon. He holds a BSc and an MSc in Computer Science from NDU, and three PhDs in Applied Mathematics, Computer Science, and Applied Statistics and Probability, all from Bircham International University through a distance learning program. He also holds two PhDs in Mathematics and Prognostics from the Lebanese University, Lebanon, and Aix-Marseille University, France. Dr. Abou Jaoudé's broad research interests are in the field of applied mathematics. He has published twenty-three international journal articles and six contributions to conference proceedings, in addition to seven books on prognostics, pure and applied mathematics, and computer science.",institutionString:"Notre Dame University - Louaize",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"4",totalChapterViews:"0",totalEditedBooks:"2",institution:{name:"Notre Dame University – Louaize",institutionURL:null,country:{name:"Lebanon"}}}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"15",title:"Mathematics",slug:"mathematics"}],chapters:null,productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"252211",firstName:"Sara",lastName:"Debeuc",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/252211/images/7239_n.png",email:"sara.d@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. 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Brass, bronze, steel, titanium, and aluminum alloys are currently the most applied metallic materials, notwithstanding the natural tendency to suffer corrosion under aggressive conditions and thus return to their original ore. To overcome issues related with economical losses and lack of safety, occasioned by metal corrosion, several protection methods have been developed, including the use of:
The use of coatings on metallic surfaces has various advantages, such as relatively low costs, environmental compatibility, and the possibility to apply them on metallic components already in use. Consequently, different kinds of protective coatings have been developed, comprising metallic, inorganic, organic, or organic-inorganic materials. The application of many metal coatings, such as chromium, zinc, nickel, aluminum, and copper, involves usually inherent pollution and toxicity-related problems. The most widely used metallic coating is zinc, commonly deposited on carbon steel by hot-dip on a molten zinc bath, process called galvanization, after which the metal substrate acquires a zinc-rich top layer with a thickness of approximately 10 μm. Inorganic coatings comprise ceramics (silica, titania, zirconia, alumina), glass, carbon, etc [1]. Although the inorganic coatings present higher corrosion resistance compared to bare substrates, they usually exhibit residual porosity and stress-induced cracks, which limit their use as efficient corrosion barrier as they allow the diffusion of corrosive species to the underlying metal [4, 5]. Organic materials such as epoxy, poly(methyl methacrylate) (PMMA), polyurethane (PU), polyesters, fluoropolymers, and related paints, combined with anticorrosive primer containing various types of pigments, are widely applied as protective coatings. This is justified by the simplicity of deposition, their dense and homogeneous structure, and consequently high corrosion resistance in aggressive environments. However, their lack of thermal stability, mechanical resistance and adhesion to metallic surfaces can result in serious restriction of their long-term stability.
Organic-inorganic hybrids stand for a class of materials formed by the combination of a polymeric and a ceramic phase, resulting in a nanocomposite material with unique properties. New functionalities result from the synergy of both components, achieved by a careful adjustment of the nature, proportion, and the type of interaction at the interface of both phases. One of the most used methodologies to produce organic-inorganic hybrid materials is the sol-gel process, which allows due to its versatility to control the structure and the functional properties. Through hydrolysis and condensations reactions, the sol-gel route allows the obtain high purity, homogeneous, and structurally tuneable materials, which have a wide range of applications such as catalysts, drug release systems, photochromic devices, biosensors, transparent insulating films, and anticorrosive coatings with excellent barrier properties [6]. The latter characteristic is related to the possibility to prepare a dense organic-inorganic network structure by linking both phases covalently on the molecular scale, and furthermore, the ability to covalently bond the inorganic phase with metallic substrates, leading to highly adherent coatings. Consequently, intense research efforts are presently focused on the development of organic-inorganic hybrid coatings in form of passive barrier layers with low permeability for corrosive species such as chloride ions, water, and oxygen.
There are various methodologies to investigate the corrosion protection efficiency of coated metals; however, the most applied electrochemical techniques are electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and chronopotentiometry. Among them, EIS allows for a deeper analysis of the electrolyte/coating/substrate systems, due to the possibility to fit the data using equivalent electrical circuits, which permit to extract important electrochemical parameters such as coating capacitance, pore resistance, double layer capacitance, charge transfer resistance, water uptake, diffusivity, among others. Additional methods like salt spray test and immersion techniques are used according to different norms for the qualitative and quantitative evaluation of corrosion zones, pitting, and for the determination of corrosion rates. To evaluate the electrochemical performance of the protective system for a given corrosive environment and coating thickness, the most important criteria are (i) the magnitude of the initial impedance modulus obtained by EIS at low frequency, defined as corrosion resistance; (ii) the values of the open circuit potential, obtained by chronopotentiometry; and (iii) the time evolution of both parameters, to evaluate the long-term stability of the coatings. For industrial application, another important aspects have to be considered such as the simplicity of the synthesis process, low costs of reagents, and their environmental compatibility.
One example of an efficient corrosion protection of mild steel was recently reported for a hybrid system combining an epoxy-siloxane topcoat with an epoxy primer containing micaceous iron oxide and zinc phosphate pigments [7]. The electrochemical measurements showed a high-impedance modulus of up to 100 GΩ cm2, remaining stable for more than 1 year in contact with 3% NaCl solution. The authors attribute the excellent protection to the high resistance of the coating against water uptake provided by suitable epoxy/primer combination and the relatively high thickness (~140 μm) of this coating system. In another recent study, Ammar et al. [8] report on high-performance hybrid coatings based on acrylic-silica polymeric matrix reinforced by SiO2 nanoparticles, applied to mild steel with a thickness of 75 μm by brush coating. EIS measurements confirmed the high-corrosion protection efficiency with an impedance modulus of more than 10 GΩ cm2, decreasing one decade after 90 days of immersion in 3.5% NaCl solution. Visuet et al. [9] obtained similar results for polyurethane/polysiloxane hybrid coatings containing TiO2 as pigment. The EIS analysis showed that coatings loaded with 10-wt% TiO2 (75 μm thick) were able to withstand 263 days, in 3.5% NaCl solution, with almost unaltered corrosion resistance of about 100 GΩ cm2. Their model proposes that the TiO2 pigment works as a charge (ionic) storage surfaces, thus enhancing the barrier property of the coating against electrolyte uptake.
The above results demonstrate that elevated anticorrosive performance is usually achieved for sophisticated barrier coatings with an average thickness in the order of dozens to hundreds micrometers. For the market, however, which aims on economic and efficient solutions, elevated thickness, and complexity of the coating system, implies elevated material costs and weight increase, issues that are hardly to be accepted, especially by the aerospace industry. In this regard, dos Santos and coauthors [10] have successfully prepared highly efficient PMMA-silica coatings having a thickness of only ~2 μm, which were able to withstand aggressive saline/acid (0.05 mol L−1 NaCl + 0.05 mol L−1 H2SO4) and 3.5% NaCl environments for up to 105 and 196 days, respectively, maintaining the corrosion resistance in the GΩ cm2 range. The excellent performance of the primer free coating was explained by the high connectivity of reticulated sub-nanometric silica domains densely interconnected by short PMMA chain segments. Another results that confirmed the viability of thin hybrid films as efficient corrosion barrier have been reported in the study of Harb et al. [11]. The authors showed that the addition of cerium (IV) salt into PMMA-silica system results in a further improvement of the corrosion resistance and durability of the coatings applied to polished carbon steel by dip-coating. The electrochemical behavior of ~1.5 μm thick films reached for a Ce/Si molar ratio of 0.7% an impedance modulus of about 10 GΩ cm2 (NaCl 3.5% solution) and remained stable within one order of magnitude for 304 days, a performance typically observed for high performance paint systems. The remarkable anticorrosive protection has been associated with the role of Ce(IV) as oxidation agent leading to an enhancement of the overall connectivity of the hybrid network, induced by the enhanced polymerization of organic and inorganic moieties.
In contrast to coating system designed as passive barrier, recent trends aim on the development of active multifunctional anticorrosive coatings with self-healing ability, high-thermal stability, and mechanical resistance, among other functionalities. Inspired by biological systems, the self-healing ability involves the complete recovery of the original properties of the material after suffering macroscopic lesions, induced by mechanical or chemical processes. Various strategies have been used to prepare self-healing coatings, usually containing an active compound, whether stored in microcapsules or incorporated into the coating. They can be activated by temperature increase, UV, pH gradient, breaking of capsules, or changes in the chemical environment [12, 13]. A number of studies report on the use of cerium salts (chloride and nitrates) and ceria nanoparticles as inhibitors, preventing corrosion by the self-healing ability in affected areas of inorganic, organic, and hybrid coatings. The resulting substantial lifetime increase is attributed to the formation of insoluble oxides and hydroxides in the corroded zones [3, 11, 14–17]. On the other hand, significant improvements of thermal and mechanical properties have been achieved by incorporation of clays, lignin, carbon nanotubes, graphene oxide, and graphene into polymeric or organic-inorganic matrices [18–20].
This chapter reports on recent results obtained for high-performance PMMA-silica and epoxy-silica hybrids coatings, correlating their structural properties with the corrosion protection efficiency, accessed by potentiodynamic polarization and electrochemical impedance spectroscopy. Moreover, several interesting finding are presented regarding PMMA-silica hybrids reinforced with lignin, carbon nanotubes, and graphene oxide to improve their thermal and mechanical properties, as well as some recent results on active corrosion inhibition by the self-healing ability of Ce(IV) containing PMMA-silica coatings.
All reagents used to epoxy-silica and PMMA-silica hybrids synthesis were purchased from Sigma-Aldrich and used as received, apart from the methyl methacrylate (MMA) monomer, which had been distilled before use to remove the polymerization inhibitor. The molecular structures of the epoxy-silica and PMMA-silica hybrid precursors are presented in Figures 1 and 2, respectively, and the synthesis procedures are summarized in Figure 3.
Molecular structures of the epoxy-silica hybrid precursors.
Molecular structures of the PMMA-silica hybrid precursors.
Synthesis procedures used to prepare epoxy-silica and PMMA-silica hybrids.
Epoxy-silica hybrids were prepared from the curing reaction of poly(bisphenol A-co-epichlorohydrin), glycidyl end-capped (DGEBA, Mn = 377 g/mol) with diethyltriamine (DETA) as hardener, and (3-glycidoxypropyl)methyltriethoxysilane (GPTMS), as coupling agent between the organic and inorganic phase, combined with the sol-gel hydrolysis and condensation reactions of tetraethoxysilane (TEOS) and GPTMS. In the first step, DGEBA and GPTMS were mixed with DETA in tetrahydrofuran (THF) solvent during 4 h at 70°C and 25 min at 25°C, under constant stirring in a reflux flask. In the next step, TEOS, ethanol, and acidified water (pH 1 using nitric acid) were added to the reflux system at room temperature and stirred for an additional 1 h. At this stage, the sol-gel reactions take place, as shown below, where the alkoxide precursors (TEOS and GPTMS) are hydrolyzed, forming Si–OH groups, Eq. (1), which subsequently condense with an initial alkoxide molecule, Eq. (2), or another Si–OH group, Eq. (3), yielding Si–O–Si bond and eliminating alcohol or water, respectively. The homogeneous and transparent sols were used for the film deposition by dip-coating onto A1020 carbon steel.
Two series of epoxy-silica hybrids were prepared, varying the amount of GPTMS or TEOS and keeping the molar concentrations of other compounds constant (Figure 4). In order to ensure a fully cured thermosetting, DETA was added in a proportion that resulted in one oxirane group for each hydrogen atom of the amine groups.
Epoxy-silica hybrids sample names and compositions.
PMMA-silica hybrids have been prepared by the radical polymerization of methyl methacrylate (MMA) and 3-(trimethoxysilyl)propyl methacrylate (MPTS, also known as TMSM) using benzoyl peroxide (BPO), as thermal initiator of the polymerization, and tetrahydrofuran (THF) as solvent. The sol-gel route has been used to perform the hydrolytic condensation of tetraethoxysilane (TEOS) and MPTS, using ethanol and acidified water (pH 1 using nitric acid), during 1 h at room temperature. In the presence of acidified water, alkoxide precursors (TEOS and MPTS) are hydrolyzed and subsequently condensed to form Si–O–Si bonds. After mixing the organic and inorganic precursor, the obtained transparent and homogeneous sols were used to deposit few micrometer thick films onto A1020 carbon steel or AA2024 aluminum alloy substrates.
To investigate the relation between structure and barrier properties, the hybrids films were prepared at different synthesis conditions (Table 1), varying the ratio between the organic to inorganic phase (MMA/TEOS), the temperature (80–100°C) and time (2–4 h) of the organic precursor reaction, as well as the BPO/MMA molar ratio (0.01–0.1). The molar ratios of H2O/Si = 3.5 and ethanol/H2O = 0.5 were kept constant. Ce(IV) salt (ammonium cerium nitrate), lignin, carbon nanotube (CNT), and graphene oxide (GO) were added separately as modifier to the inorganic precursor of the PMMA-silica hybrid.
Samples | MMA:MPTS:TEOS molar ratio | BPO/MMA molar ratio | Organic phase synthesis | Filler | Reference |
---|---|---|---|---|---|
M2 | 2:1:2 | 0.01 | 70°C/2 h | – | [22] |
M4 | 4:1:2 | 0.01 | 70°C/2 h | – | [22] |
M8 | 8:1:2 | 0.01 | 70°C/2 h | – | [22] |
M10 | 10:1:2 | 0.01 | 70°C/2 h | – | [22] |
M8_4h | 8:1:2 | 0.01 | 80°C/4 h | – | [18] |
M8_4h_E0.2 | 8:1:2 | 0.01 | 80°C/4 h | [10] | |
M8_T80B0.01 | 8:1:2 | 0.01 | 80°C/4 h | – | – |
M8_T90 | 8:1:2 | 0.01 | 90°C/4 h | – | – |
M8_T100 | 8:1:2 | 0.01 | 100°C/4 h | – | – |
M8_B0.05 | 8:1:2 | 0.05 | 80°C/4 h | – | – |
M8_B0.10 | 8:1:2 | 0.10 | 80°C/4 h | – | – |
M8_Ce | 8:1:2 | 0.01 | 70°C/2 h | Ce/Si molar ratio: 0.1, 0.2, 0.3, 0.5, 0.7, 1, 3, 5% | [11] |
M8_lignin | 8:1:2 | 0.01 | 70°C/2 h | lignin: 0.05, 0.10, 0.50, 1.00 wt.% | [20] |
M8_CNT | 8:1:2 | 0.01 and 0.05 | 80°C/4 h | CCNT/Si molar ratio: 0.05% | [18] |
M8_GO | 8:1:2 | 0.01 and 0.05 | 80°C/4 h | CGO/Si molar ratio: 0.05% | [18] |
PMMA-silica hybrid preparation conditions.
Carbon steel 1020 (25 mm × 20 mm × 5 mm), a ferrous alloy with low carbon content, and 2024 aluminum alloy (20 mm × 20 mm × 1 mm) have been used as substrates. Low-carbon steels are produced in large quantities at relatively low costs and widely used in automobilist, construction, oil industries, etc [21]. Although the use of ferrous alloys is economically viable due to the low cost and versatility, corrosion is the great obstacle when it comes to the durability of these materials that undergo severe corrosion in contact with humid environments, low amounts of chloride ions and acid solutions in general. The 2000 and 7000 series of aluminum alloys, containing roughly 4.3–4.5% copper, 0.5–0.6% manganese, 1.3–1.5% magnesium, are widely used in the aerospace industry due to their improved mechanical properties; however, they are susceptible to enhanced corrosion especially at the grain boundaries. Prior to deposition, all substrates had been sanded with 100, 300, 600, and 1500 grit emery paper, washed with isopropanol for 10 min in an ultrasound bath and dried under a nitrogen stream. The deposition of the hybrids coatings was performed by dip-coating (Microchemistry—MQCTL2000MP) at a rate of 14 cm min−1, with 1 min of immersion and air-drying during 10 min at room temperature. This procedure was performed three times for each sample. The coated substrates and the remaining solution, placed in Teflon holders, were cured for 24 h at 60°C and then 3 h at 160°C to ensure the liberation of all volatile species and the densification of the hybrid matrix.
Structural and morphological characteristics have been investigated using nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), small angle X-ray scattering (SAXS), atomic force microscopy (AFM) and thermogravimetric analysis (TGA). The anticorrosive properties of coated samples were evaluated by exposure of the coated samples to standard 3.5% saline and saline/acid solutions, using electrochemical impedance spectroscopy (EIS).
The thickness of the coatings was determined using a Filmetrics F3-CS optical interference system. An Agilent Technologies Model 5500 atomic force microscope was used to obtain AFM topography images, in tapping mode, with 1 × 1 μm, of the hybrid coatings deposited on the metallic substrates. 29Si nuclear magnetic resonance spectroscopy (29Si-NMR) measurements of the hybrid powders were performed in a 300-MHz Varian Inova spectrometer, using a Larmor frequency of 59.59 Hz and tetramethyl silane (TMS) as an external standard. The CasaXPS processing software was used for spectral deconvolution using Gauss profiles. XPS was carried out in a UNI-SPECS UHV surface analysis system, using the Mg Kα radiation (hν = 1253.6 eV) and pass energy of 10 eV to record the high-resolution spectra. The near surface composition was determined from relative peak intensities of carbon (C 1s), oxygen (O 1s) and silicon (Si 2p) corrected by Scofield’s atomic sensitivity factor of the corresponding elements. To study the oxidation state of Ce (Ce 3d) and the local bonding structure of carbon (C 1s), oxygen (O 1s), and silicon (Si 2p) of the coatings, the spectra were deconvoluted applying Voigt profiles and Shirley’s background subtraction using the CasaXPS software. SAXS experiments were carried out at the SAXS-1 beamline in the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil) to determine the nanostructural characteristics of the hybrids. The scattering intensity I(q) was recorded as a function of the modulus of the scattering wave vector q = (4π/λ) sin θ, θ being half of the scattering angle. The SAXS beamline uses a monochromatic X-ray beam (λ = 1.548 Å) and a 2D detector, Dectris Pilatus 300k, positioned 0.9 m away from the sample holder. Thermogravimetric analysis of unsupported hybrids films was performed in a TA Instruments STD Q600 analyzer, under a nitrogen flow of 100 mL min−1.
The anticorrosive performance of hybrid coatings, deposited on A1020 carbon steel or Al2024 aluminum alloy, was investigated by electrochemical impedance spectroscopy (EIS) with a Gamry Potentiostat Reference 600, using 10 points per decade and RMS amplitude of 10 mV in a frequency range of 50 mHz–100 kHz. The electrochemical cell consisted of an Ag|AgCl|KClsat reference electrode, a platinum mesh counter electrode, a platinum electrode connected to the reference electrode through a 0.1-μF capacitor and the working electrode of either coated or uncoated metal substrate. The measurements were performed once a week, in saline (3.5% NaCl) or saline/acid solutions (0.05-mol L−1 H2SO4 + 0.05-mol L−1 NaCl), until a significant drop of the impedance modulus was observed, indicating the failure of the coating.
A number of interesting results have been obtained for novel epoxy-silica and PMMA-silica hybrid coatings, concerning their nanostructural properties, modified by the variation of synthesis conditions or by addition of nanofillers, in form of lignin, carbon nanotubes, and graphene oxide. The main purpose of this work was to relate these properties with the barrier characteristics, in terms of corrosion resistance and durability in aggressive environments and to compare the obtained results with those reported for a variety of hybrid coating systems. For the fine tuning of the performance of both coating systems toward an efficient and stable anticorrosive barrier, it is crucial to obtain detailed information on the formation process of the hybrid network and the structural and compositional properties of the nanocomposites.
Bisphenol stands for a group of chemical compounds with two hydroxyphenyl functionalities. There is a wide diversity of bisphenol molecules; however, the most common are the Bisphenol A (BPA) and the Bisphenol F (BPF) (Figure 5). Epoxy resins can be produced from the combination of bisphenol, such as bisphenol A, with epichlorohydrin (IUPAC name: 2-(chloromethyl)oxirane)) to give, for example, bisphenol A diglycidyl ether (Figure 6). The epoxy resins present in general poor thermal, mechanical, and chemical stability, properties which are however significantly improved when a curing agent is added. Most curing agents are composed of nitrogen-containing molecules that have a functionality equal or superior of three (f ≥ 3), which provides cross-linking between the bisphenol segments. The functionality is the number of available bonding sites, such as f = 4 for diamino diphenyl methane (4 hydrogens prone to provide bond), f = 6 for triethylene tetraamine, and f = 5 for diethylenetriamine (Figure 7). Curing reactions by DETA proceed by SN2 nucleophilic attack of the curing agent to the less-substituted carbon in the oxirane ring, resulting in its opening and formation of an OH group. The nitrogen of the amine group can attack another epoxy ring resulting in a highly branched polymer system, known as a thermoset, which presents high thermal stability and mechanical resistance [23]. This second nucleophilic attack of nitrogen can occur at the epoxy group of the resin or another molecule containing epoxy group, such as (3-glycidoxypropyl)trimethoxysilane (GPTMS) to produce an organic-inorganic hybrid structure (Figure 8).
Common epoxy resin precursors.
Bisphenol A diglycidyl ether.
Common curing agents.
Molecular structure of the epoxy-silica hybrid.
Simultaneously to the curing reaction, the sol-gel reactions of hydrolysis and condensation take place to produce the silica inorganic phase. GPTMS and TEOS Si–O–R groups, in presence of acidified water, become Si–OH through the hydrolysis reaction, and posteriorly, the Si–OH groups can condense with another Si–OH group or an initial Si–O–R group, forming Si–O–Si bonding and eliminating water or alcohol, respectively.
The surface characterization of the epoxy-silica hybrids deposited on carbon steel has shown that the coatings are uniform, transparent, smooth, and crack free (Figure 9). AFM images with an area of 1 μm2 were used to obtain the surface roughness of the coatings. Table 2 summarized all RMS surface roughness and thickness values determined for epoxy-silica hybrids of T-series (TEOS variation) and G-series (GPTMS variation). With increasing GPTMS and TEOS fraction, a significant increment of the surface roughness can be observed. The data suggest that increasing concentration of TEOS has a larger impact on the surface roughness than that of GPTMS, probably due to the formation of silica domains of larger size. Measurements of the films thickness indicate for all samples of the G-series a constant value of about 1.7 μm, while for films of the T-series the thickness varies from 2 to 3 μm, except for the T1.5 sample having 6.7 μm.
Representative image (a), and AFM image (b), of T1/G1 epoxy-silica hybrid coating deposited on carbon steel.
Samples | Thickness (μm) | RMS roughness (nm) | Cd (%) | α | Rg (nm) | d (nm) | T0 (°C) | EIS lifetime (days) |
---|---|---|---|---|---|---|---|---|
T0 | 3.0 | 0.3 | – | – | – | 3.7 | – | – |
T0.5 | 2.0 | 0.7 | 85.0 | 3.8 | – | 4.2 | 306 | 2 |
T1/G1 | 1.6 | 1.2 | 83.8 | 3.2 | 0.8 | – | 293 | 5 |
T1.5 | 6.7 | 0.3 | 87.1 | 3.9 | 0.8 | – | 314 | 42 |
T2 | 3.3 | 2.6 | 87.8 | 3.5 | 0.8 | – | 295 | 1 |
T2.5 | 1.8 | 5 | – | 3.4 | 0.8 | – | 302 | 4 |
G0.0 | 1.0 | – | – | 3.8 | – | – | – | – |
G0.5 | 1.8 | 0.6 | 87.8 | 1.8 | 1.5 | – | 285 | 55 |
G1.5 | 1.8 | 0.4 | 94.7 | 4.0 | 0.6 | – | 297 | – |
G2 | 1.6 | 1.0 | – | 4.0 | 0.4 | – | 304 | 1 |
G2.5 | 1.8 | 1.4 | – | 4.0 | 0.3 | – | 306 | 2 |
Properties of epoxy-silica hybrids: film thickness (optical interferometry); surface roughness (AFM); degree of polycondensation, Cd, (29Si-NMR); Porod coefficient, α, radius of gyration, Rg, and correlation distance, d, (SAXS); temperature of the limit of thermal stability T0 in N2 atmosphere (TGA); and coating lifetime in 3.5% NaCl (EIS).
Table 3 shows that results of the quantitative XPS analysis are in good agreement with those obtained for the nominal composition for both series of samples. As expected, the data show an increase of silicon and oxygen atomic concentration for the G and T-series, while nitrogen content increases slightly only for the G-series due to the higher DETA content. As the structure of GPTMS contains also carbon atoms, its addition leads to a less pronounced increase of the Si content. As a consequence, for the G-series the decrease of the C/Si ratio from 14.1 (G0.5) to 10.0 (G2.5) was smaller than that observed for the T-series from 15.9 (T0.5) to 8.3 (T2.5).
G0.5 | G2.5 | T0.5 | T2.5 | |||||
---|---|---|---|---|---|---|---|---|
XPS | Nominal | XPS | Nominal (at.%)* | XPS | Nominal | XPS | Nominal | |
Si 2p | 4.8 | 4.3 | 6.3 | 6.0 | 4.4 | 4.0 | 7.5 | 7.6 |
C 1s | 67.7 | 71.0 | 62.7 | 66.0 | 70.1 | 71 | 62.1 | 61.8 |
O 1s | 25.5 | 22.0 | 28.1 | 25.0 | 22.9 | 22.3 | 28.1 | 28.1 |
N 1s | 2.0 | 2.5 | 2.8 | 3.5 | 2.5 | 3 | 2.4 | 2.6 |
C/Si | 14.1 | 16.9 | 10.0 | 11.0 | 15.9 | 19 | 8.3 | 8.1 |
Comparison between XPS and calculated nominal atomic concentrations for epoxy-silica coatings.
*XPS experimental error ±5%.
The chemical bonding structure of the inorganic network can be characterized according to the proportion of different Si species having a fixed number of oxygen bridging silicon atoms bonded to one (central) silicon atom. A common notation is Qj for orthosilicates (0 ≤ j ≤ 4), such as TEOS, and Tj for organically modified silicates (0 ≤ j ≤ 3), such as GPTMS, where j gives the number of Si–O–Si bridges attached to the silicon atom.
Figure 10 shows the 29Si NMR spectra, fitted with Gaussian components, used to extract the proportion of Qj and Tj species. It can be observed that the Q4 and T3 peaks (−107 ppm and −62 ppm, respectively) have the highest intensities in relation to the other components related to lower network connectivity. The degree of connectivity of the inorganic phase, the so-called degree of polycondensation, Cd, has been determined from the fitted 29Si NMR spectra using the following equation:
29Si-NMR spectra obtained for epoxy-silica hybrids. Inset: schematic representation of the TJ and QJ species, where ‘R’ indicates OH or OCH3 or OCH2CH3 groups.
The Cd values of Table 3 show a high connectivity of the inorganic network with a clear predominance of a tetra-substituted TEOS and a tri-substituted GPTMS sites. Furthermore, it seems that an increase of GPTMS favors the Q4 and T3 structures, yielding a highly cross-linked inorganic network reaching about 95% connectivity for G1.5 sample, while the variation of TEOS does not change the Cd values significantly, remaining in the range of 85–88%. More information on the structure and size of the inorganic domains was obtained by small angle X-ray scattering (SAXS) measurements.
The SAXS technique allows to access the nanostructural characteristics of the inorganic network due to the higher electronic density of silica compared that of the polymeric matrix. The log-log plots of scattering intensities I (q) recorded for different fractions of GPTMS and TEOS (Figure 11) show three main characteristics: a linear decay located at low q values, corresponding to the Porod region; a Gaussian decay in the mid q-range, corresponding to the Guinier regime; and a broad correlation peak superimposed to the Guinier region, observed only for T0.0 and T0.5 samples. The former feature, in the mid q-range, is characteristic of a diluted set scatters, while the latter is the result of the interferences of the scattered X-ray caused by the concentrated set of nano-objects.
SAXS curves of T-series (a), and G-series (b), where the black lines represents the fits used to calculate the Porod coefficient (α) for q < 1 (except for the G0.5 sample, which presents α at q>1), and the radius of the gyration (Rg) for q >1. (The intensities were shifted to obtain a better visualization of the curves).
These scattering patterns have been already observed for other silica-polymer hybrids [24, 25] and attributed to a hierarchical organization of silica nano-domains. Accordingly, we propose that the nanostructure of the hybrid can be described by a two-level hierarchical model, corresponding to a diluted or concentrated (T0.0 and T0.5) set of silica nanoparticles inside the aggregation zones embedded in the polymer matrix. In the case of the diluted system, the size of the smaller particles was determined, in terms of the radius of the gyration, Rg, by fitting the Gaussian decay observed in the mid q-range using the Guinier model: I(q) = I0 exp (−Rg q2/3), where I0 is the scaling factor. However, this was only possible for scattering curves, which did not present an overlapping correlation peak. Therefore, values for Rg and those for the correlation distance, d ≈ 2π/q, have been obtained only for a restricted number of samples (Table 2). Except for T0.0, the form of the scattering objects was determined by fitting the curves using the Porod model: I ∝ q−α, where α is the Porod exponent. α ≈ 4 indicates a bi-phase system formed by set of nearly isometric scattering objects with a smooth surface, while for smaller values, a rough surface (fractal) is expected.
The results indicate that the inorganic phase consists of aggregates with relatively smooth surface and an average spacing of several nanometers (d ≈ 4 nm). These domains have been formedby agglomeration of smaller silica particles with a size of about 1 nm (0.3 < Rg < 1.5 nm).
Some clear correlations between these parameters and the increasing silica concentration of the G- and T-series could be established. For the G-series, the evolution of the SAXS pattern evidences the role of GPTMS in controlling the size of primary silica particles and they aggregation. The power law decay over a decade and α = 3.8 observed for the hybrid prepared only with TEOS (G0.0) characterizes the scattering by the surface of very large silica particles (>30 nm). The addition of a small amount of GPTMS (G0.5 sample) reduces the size of silica particles more than ten times (Rg = 1.5 nm) and prevents further aggregation, as evidenced by extended plateau at q < 0.1 nm. These unique features suggest for G0.5 sample an elevated nanostructural homogeneity, which might be responsible for the superior corrosion protection performance of this material (Table 2). In the case of the T-series, the correlation peak disappears for higher TEOS content and the linear decay shifts to higher q-values. These features evidence that TEOS addition favors the formation of more open aggregates, leading to a less compact nanostructure.
The thermal properties of the hybrids were studied by thermogravimetry under nitrogen flow. Table 2 shows the temperature of the limit of thermal stability, T0, for all epoxy-silica hybrids, defined as the temperature of 5% weight loss during the annealing process. The hybrids presented a thermal stability of about 300°C, relatively high values compared with those of other polymeric and hybrid materials [18, 22]. This advantageous property comes from the highly cross-linked structure provided by the curing agent (DETA) combined with the high polycondensation degree of the silica phase, as revealed by 29Si NMR.
Samples | Thickness (μm) | RMS roughness (nm) | Cd (%) | T0 (°C) | |Z| (GΩ cm2) | EIS lifetime (days) | Reference |
---|---|---|---|---|---|---|---|
M2 | 1.5 | – | 80.9 | – | ~0.001 | – | [21] |
M4 | – | – | 79.7 | – | ~0.01 | – | [21] |
M8 | 3 | 0.3 | 83.9 | – | ~1 | 18 | [21] |
M10 | – | – | 75.8 | – | ~0.001 | – | [21] |
M8_4h | 2.8 | 0.4 | 78.0 | 205 | ~0.1 | 56 | [18] |
M8_4h_E0.2 | 2.0 | 0.5 | 82.0 | 230 | ~3 | 196 | [10] |
M8_T80B0.01 | 2.8 | – | – | 230 | ~3 | 40 | – |
M8_T80B0.01* | 3.0 | – | – | – | ~50 | 560 | – |
M8_T90 | 2.6 | – | – | 238 | ~5 | 34 | – |
M8_T100 | 2.5 | – | – | 250 | ~0.1 | 35 | – |
M8_B0.05 | 5.0 | – | – | 270 | ~10 | 42 | – |
M8_B0.10 | 9.7 | – | – | 223 | ~10 | >186 | – |
Properties of PMMA-silica hybrid coatings: film thickness (optical interferometry); surface roughness (AFM); degree of polycondensation, Cd, (29Si-NMR); limit of thermal stability T0 in N2 atmosphere (TGA); impedance modulus |Z|, after 1 day exposure to 3.5% NaCl solution (EIS); and coating lifetime in 3.5% NaCl (EIS).
*Al2024 substrate.
The anticorrosive performance of the hybrids was assessed by EIS measurements, in a 3.5% NaCl saline solution at 25°C. The hybrid coatings deposited on carbon steel were attached to an electrochemical cell, and after verifying a constant value of the open-circuit potential, the impedance measurements were performed as a function of time until a significative drop of the impedance modulus occurred. This time period was defined as lifetime of the coating, listed in Table 2. The impedance modulus at low frequency of the Bode plot is generally used as an indicator of the anticorrosive performance of the coating, with values higher than 0.1 GΩ cm−2 typically considered an excellent protection. The corrosion resistance of the films generally decreases with time, caused by the penetration of electrolyte into the protective layer through zones of residual porosity and defects.
The Nyquist and Bode plots obtained after 1 day of immersion in 3.5% NaCl solution are presented in Figure 12. It can be observed that two samples containing intermediate TEOS to GPTMS ratios (T1.5 and G0.5) presented the highest impedance modulus of 0.9 and 0.2 GΩ cm2, respectively, and showed also the longest lifetime of several weeks (Table 2). These coatings show at higher frequencies (>1 Hz), a capacitive behavior with a phase angle higher than −80° extending over a range of 4 decades, characteristic for an efficient anticorrosive barrier layer. In contrast, for formulations with excess of TEOS or GPTMS, both the corrosion resistance and lifetime values show considerably lower values.
Nyquist and Bode plots of (a) series T and (b) series G of the epoxy-silica coatings deposited on carbon steel, compared to those of bare carbon steel, after 1 h of immersion in 3.5% NaCl solution.
The results of the structural analysis indicate that the excellent barrier properties, found for coatings with intermediate TEOS to GPTMS ratio, result from a highly reticulated hybrid structure combining a number of favorable properties, such as a high polycondensation degree of the inorganic phase, a extremely smooth surface, indicating a very homogeneous distribution of silica nanodomains, high thermal stability, as well as an adequate quantity of the silica phase which ensures a good adhesion of the film to the metallic substrate. Although the corrosion protection efficiency of the best epoxy-silica coatings, reported so far [7, 26, 27], is comparable with results presented in this work, it may profit from their 10–100 times higher thickness.
Poly(methyl methacrylate), also known as acrylic and Pexiglas®, is a rigid, low cost, nontoxic, transparent and colorless thermoplastic polymer, extensively used as optical lenses, protective coatings, optical fibers, and as an alternative to glass in windows as well as a variety of household appliances. The introduction of an inorganic component, such as silica, improves the thermal stability, mechanical strength, and the adhesion to metallic substrates, the latter property being an essential feature for a high-efficiency coatings. The covalent bond between the PMMA and the silica phase can be achieved by the addition of 3-(trimethoxysilyl)propyl methacrylate (MPTS), a coupling agent formed by an alkoxy-silane group attached by a nonhydrolysable Si–C bond to the acrylic tail, which polymerizes with PMMA chains, while the inorganic part reacts with the silica precursor (TEOS), yielding an organic-inorganic hybrid structure, shown in Figure 13.
Molecular structure of the PMMA-silica hybrid.
A variety of PMMA-silica hybrids have been studied, changing the organic/inorganic phase proportion, the amount of thermal initiator, the synthesis temperature and time, as well as the ethanol-to-water ratio. Furthermore, cerium salt has been added to the PMMA-silica matrix as corrosion inhibitor, and lignin, carbon nanotubes, and graphene oxide as fillers. The main results, found for pure PMMA-silica hybrids, are summarized in Table 4, those obtained using additives will be discussed in the following sections.
Similar to epoxy-silica coatings, PMMA-silica hybrids deposited on metallic substrates were homogeneous, transparent and had a very smooth surface. Structural analysis of PMMA-silica hybrids, performed by AFM, SAXS, NMR and XPS, has shown that the nanostructure is formed by a dense amorphous network of ramified silica-siloxane cross-link nodes, covalently interconnected by short PMMA chain segments [10, 22]. Varying the MMA/MPTS molar ratio from 2 to 10, NMR and SAXS results have shown that the M8 sample (MMA/MPTS = 8) presented the highest degree of polycondensation (83.9%) of the silica nanoparticles with an average radius of 0.8 nm, spaced by PMMA segments over an average distance of 4.6 nm. This coating exhibited also an excellent adhesion to the substrate (detachment force > 3.5 MPa) and the best anti-corrosion performance [10, 22]. EIS and potentiodynamic polarization results have shown that the M8 coating deposited on carbon steel acts as a very efficient corrosion barrier, increasing the total corrosion resistance by almost 6 orders of magnitude (>1 GΩ cm2) and reducing the current densities by more than 4 orders of magnitude (<0.1 nA cm−2), compared to the bare steel substrate [22]. Furthermore, XPS analysis confirmed that no corrosion-induced changes had occurred after 18 days of immersion in 3.5% NaCl solution [22].
Increasing the synthesis temperature from 70 to 80°C and the time of reaction from 2 to 4 h (sample M8_4h), an increase in the amount of polymeric phase was detected yielding a more compact and durable coating (56 days) [18, 22]. After optimizing also the ethanol-to-water ratio of the inorganic phase to a value of 0.2, the corrosion resistance and lifetime were further increase to 196 days in 3.5% NaCl (M8_4h_E0.2 coating, Table 4) [10]. Other important finding was the improvement of the corrosion resistance by hot deposition (M8_T80B0.01), which enhances the reaction between the inorganic phase and the metal substrate, improving the coating adhesion. This sample has been also deposited on Al2024 substrate and tested in saline (Figure 14a) and saline/acid environment (Figure 14b). This coating highlights a corrosion resistance of up to 50 GΩ cm2, in saline environment, showing only a small performance decrease to 0.1 GΩ cm2 after 560 days exposure. This is to our best knowledge the highest durability, obtained so far for hybrid coatings in standard saline solution. Also in contact with a saline/acid solution (0.05 mol L−1 H2SO4 + 0.05 mol L−1 NaCl), this about 3-μm thick coating presented a high corrosion resistance (20 GΩ cm2), remaining almost unchanged during its lifetime of 87 days. It is interesting to note that the phase angle dependence has a capacitive behavior (θ ≈ −90°), over a frequency range of 6 decades, a behavior close to that of an ideal capacitor, highlighting the extraordinary performance of this coating.
Time dependence of Nyquist and Bode plots of the M8_T80BPO0.01 PMMA-silica coating deposited on Al2024 substrate in contact with a) in 3.5% NaCl solution and b) 0.05 mol L−1 NaCl + 0.05 mol L−1 H2SO4 solution.
Molecular structure of (a) lignin, (b) carbon nanotube, and (c) graphene oxide.
PMMA-silica hybrids have been also prepared at different synthesis temperatures of the organic precursor (80–100°C) and different BPO/MMA molar ratio (0.01–0.1), using the well-established MMA/MPTS molar ratio of 8 [22]. The increase in the synthesis temperature did not influence significantly the structure, the thermal properties and the corrosion resistance, however, the increase of the BPO amount led to an increase of the polymerization degree, thermal stability of 40°C (BPO0.05), and also of the anticorrosive efficiency (Table 4). The sample M8_BPO0.05 and M8_BPO0.10 presented an impedance modulus of 10 GΩ cm2 in a saline medium (3.5% NaCl), remaining essentially unchanged during more than 6 months of exposure (M8_BPO0.10).
After identifying the optimum proportion of polymeric and silica phase for the formation of a highly ramified structure (MMA/MPTS/TEOS molar ratio = 8/1/2), increasing molar percentage of Ce(IV) ions (0.1% < Ce/Si < 5%) have been added to the inorganic precursor to enhance the passivating character of the films [11].
NMR, XPS, and SAXS results, summarized in Table 5, have revealed the active role of Ce(IV) in the PMMA-silica matrix. The correlation of XPS and NMR data evidenced that the Ce(IV) concentration is directly related to the polycondensation degree (Cd) and the degree of Ce(IV) reduction, both decreasing with increasing cerium concentration. Low concentrations of cerium lead to an enhanced polycondensation of the siloxane/silica phase, with connectivity of the inorganic phase up to 87%. For low doping levels of Ce/Si < 0.7%, SAXS results have revealed increasing values of radius of gyration, Rg, suggesting an active role of Ce(IV) as oxidation agent in the enhanced growth of a cross-linked and polycondensed inorganic phase. Detailed investigation of the structural effects of cerium species has shown that reduction of Ce(IV) ions not only catalyzes a higher connectivity of the silica phase, but also enhances the polymerization of organic moieties. The resulting enhancement of the overall network connectivity leads to an improvement of the thermal stability of the hybrids, as evidenced by the results of the thermogravimetric analysis [11].
Properties | Ce(IV)/Si molar fraction (%) | ||||||||
---|---|---|---|---|---|---|---|---|---|
0 | 0.1 | 0.2 | 0.3 | 0.5 | 0.7 | 1 | 3 | 5 | |
Cd (%) | 82.8 | 87.1 | – | 83.6 | 82.7 | 79.3 | 78.5 | 77.6 | 77.3 |
XPS Ce(IV) fraction (%) | – | – | – | 28.5 ± 4 | 37.2 ± 3 | 46.4 ± 2.5 | 48.5 ± 2.5 | 55.5 ± 2 | 60.4 ± 1.5 |
Rg (nm) | 0.9 | 1.1 | 1.8 | 2.3 | 2.3 | 1.9 | 1.9 | – | – |
|Z| (GΩ cm2) | ~0.5 | ~0.5 | – | – | ~0.1 | ~10 | ~5 | – | ~0.5 |
Lifetime (days) | 42 | 85 | – | 96 | – | 304 | 65 | 48 | 404 |
Properties of PMMA-silica hybrids containing different amounts of Ce(IV): degree of polycondensation, Cd, (29Si-NMR); percentage of the Ce(IV) oxidation state (XPS Ce 3d spectra); radius of gyration, Rg, (SAXS); and impedance modulus, |Z|, after 1 day exposure to 3.5% NaCl solution (EIS).
The electrochemical assays, performed by EIS and potentiodynamic polarization curves, have shown that the PMMA-silica coatings containing intermediate concentrations of cerium present a combination of high-corrosion resistance (~10 GΩ cm2), elevated overpotential stability at low-current densities (<10−11 A), as well as excellent long-term stability of up to 304 days. Compared to the bare carbon steel substrate, the coated samples showed up to 6 orders of magnitude higher impedance modulus and up to 6 orders of magnitude lower current densities [11].
For coatings containing elevated Ce(IV) doping levels (Si/Ce = 5%), the self-healing effect was observed, induced by the formation of insoluble cerium oxides and hydroxides in corrosion affected regions. The presence of these phases in the near surface region was evidenced by XPS O 1s spectra and by scanning electron microscopy, showing the presence of nanopits (<300 nm). It was suggested that these phases were formed by reactions of Ce(III) and Ce(IV) with water and residual hydroxyl groups of the hybrid. The self-healing process prevented the progression of the corrosion process for more than 13 months keeping the corrosion resistance constant above 0.01 GΩ cm2. The excellent anticorrosive efficiency achieved by PMMA-silica coatings containing cerium can be related to a double effect of Ce(IV), combining the densification of the hybrid network with the formation of insoluble cerium species in regions affected by pitting [11].
Lignin is a macromolecule present in the cell walls of terrestrial plants that confers rigidity and impermeability, usually corresponding to 15–30% of the dry weight of wood (Figure 15a). Presently, millions of tons of lignin are generated from biodiesel and ethanol production, and most part is incinerated to generate electric energy. However, nobler applications have been found to add value to this biomass, such as the reinforcement of different classes of materials. Properties, such as low density, low abrasive character, hydrophobicity, and low cost, make lignin ideal to use as filler in polymeric and organic-inorganic hybrid matrices [20].
Carbon nanotubes (CNTs) and graphene oxide (GO) (Figure 15b and c) are also very interesting nanofillers for the structural reinforcement of polymeric and hybrid materials, due to their exceptional thermal, chemical, and mechanical resistance. Both present a hexagonal sp2 arrangement of carbon atoms, forming extremely stable cylindrical and monolayer structures, respectively. Graphene oxide has been obtained from oxidation and exfoliation of graphite, yielding a graphene layer containing oxygen functional groups such as epoxy, hydroxyl, and carboxyl [18].
PMMA-silica hybrids reinforced with 0.05, 0.10, 0.50 and 1.00 wt.% of lignin have been deposited on carbon steel by dip-coating, producing about 2.5 μm thick coatings (Table 6). Optical microscopy and optical microscopy and atomic force microscopy showed that lignin was well dispersed in the hybrid matrix, and all coatings presented a low RMS surface roughness between 0.3 and 0.4 nm. The introduction of lignin in the PMMA-silica hybrid increased the water contact angle of the film surface from 79.3° to 87.9°, the hardness from 22.9 to 30.9 HV, and the scratch resistance (critical load for delamination) from 55 to 80 mN. In addition, the thermal degradation events, obtained by thermogravimetric analysis (TGA) under nitrogen atmosphere, were shifted to higher temperatures with lignin addition, due to its phenolic structure which has the ability to trap radicals formed during the depolymerisation. Besides increasing the thermal stability of the polymeric phase, it acts also as UV stabilizer [20].
Sample | Thickness(μm) | T0 (°C) | Critical load (mN) | |Z| (GΩ cm2) | EIS lifetime (days) | Reference |
---|---|---|---|---|---|---|
M8 (70°C/2 h) | 2.4 | 170 | 55 | ~0.1 | 18 | [20] |
M8_lignin_0.50 wt.% | 2.6 | 180 | 65 | ~0.5 | – | [20] |
M8_lignin_0.10 wt.% | 2.5 | 200 | 80 | ~0.5 | 50 | [20] |
M8_lignin_0.50 wt.% | 2.7 | 190 | 66 | ~0.01 | – | [20] |
M8_lignin_1.00 wt.% | 2.7 | 195 | 56 | ~0.005 | – | [20] |
M8_BPO001(80°C/4 h) | 2.8 | 205 | 78 | ~0.5 | 56 | [18] |
M8_BPO001_CNT_0.05% | 5.7 | 220 | – | ~0.1 | 43 | [18] |
M8_BPO001_GO_0.05% | 3.1 | 275 | 94 | ~1 | 203 | [18] |
M8_BPO005(80°C/4 h) | 3.5 | 208 | 84 | ~1 | 21 | [18] |
M8_BPO005_CNT_0.05% | 6.6 | 209 | 133 | ~10 | 7 | [18] |
M8_BPO005_GO_0.05% | 5.5 | 216 | 148 | ~5 | 168 | [18] |
Properties of PMMA-silica hybrids containing lignin, CNT or GO: film thickness (optical interferometry); limit of thermal stability T0 in N2 atmosphere (TGA); critical load for delamination (microscrach test); impedance modulus |Z|, after 1 day exposure to 3.5% NaCl solution (EIS); and coating lifetime in 3.5% NaCl (EIS).
The electrochemical tests performed by EIS showed that the PMMA-silica coatings containing lignin act as efficient diffusion barriers, with corrosion resistance higher than 0.1 GΩ cm2 after exposure to 3.5% NaCl aqueous solution. For intermediate lignin content of 0.10 wt.% the coatings presented best results with an impedance modulus of 0.5 GΩ cm2, remaining almost unchanged after 50 days of exposure to aggressive environment [20].
Recent studies on the inclusion of carbon nanotubes and graphene oxide in hybrid and polymer matrices have shown excellent results in terms of increased mechanical strength, scratch and wear resistance, thermal stability, adhesion to metallic substrate, hydrophobicity, and electrical conductivity [18, 19, 28–30]. Despite all of these advances, a simultaneous improvement not only of mechanical and thermal stability but also of anticorrosive efficiency of protective coatings has been accomplished only recently by the incorporation CNT and GO in a PMMA-silica matrix [18].
To synthesize the CNT and GO modified PMMA-silica hybrids, first, the single-walled carbon nanotubes and graphene oxide were dispersed in a water/ethanol, adding in the case of CNTs dodecyl sulfate surfactant (SDS) as dispersant. Then, the carbon nanostructures were added to the inorganic precursor solution of the PMMA-silica hybrid at a carbon (CNT or GO) to silicon (TEOS and MPTS) molar ratio of 0.05% in two different matrices, prepared at BPO/MMA molar ratios of 0.01 and 0.05. As the function of BPO as a thermal initiator is to produce radicals that initiate the polymerization process of MMA, an increased BPO content results in enhanced polymerization degree in the hybrid. The transparent hybrids deposited on A1020 carbon steel substrates by dip coating presented thickness values between 2.8 and 6.6 μm (Table 6), a good dispersion of the carbon nanostructures, and a very smooth surface (0.3–0.5 nm RMS surface roughness) [18].
Hybrid | Synthesis | Substrate | Deposition/thickness (μm) | EIS: |Z| (GΩ cm2), lifetime (days), solution | PolarizationIcorr (A cm−2)Ecorr (V), reference electrode | Reference |
---|---|---|---|---|---|---|
Epoxy-GPTMS-TEOS | Sol-gel | Carbon steel | Dip-coating/6.7 | ~1 42 3.5% NaCl | _ −0.08 Ag/AgCl | [This work] |
Epoxy-APTES-ZnO | Sol-gel | Mg alloy AZ31 | Dip-coating/~12 | ~1 35 0.05 M NaCl | _ | [33] |
Epoxy-APTES | _ | Carbon steel | Spray/125 | ~100 21 0.1 M Na2SO4 | _ | [27] |
Epoxy-SiO2 | Sol-gel | Mg alloy | Dip-coating/- | ~100 7 3.5% NaCl | _ | [34] |
Epoxy-APTES | Solution intercalation method | Mild steel | Brush method/70–80 | ~10 30 3% NaCl | _ | [35] |
Epoxy-APTES-tetrathiol | Sol-gel | Al alloy AA2024-T3 | Single blade/150 | ~1 350 0.5 M NaCl | _ | [26] |
Epoxy-polysiloxane | Commercial | Cold rolled low carbon steel | Air-less spray/70 | ~100 467 3% NaCl | _ −0.65 SCE | [7] |
GPTMS-MTEOS-TEOS | Sol-gel | Al alloy AA2024-T3 | Dip-coating/25 | ~0.1 38 5% NaCl | _ | [36] |
Epoxy-APTES | _ | Mg alloy AZ31 | Dip-coating/14 | ~10 31 0.5 M NaCl | _ | [37] |
Epoxy-GPTMS-MTEOS/- | Sol-gel | Al alloy AA2024 | Dip-coating/~8 | ~1 51 0.05 M NaCl | 10−10 −0.3 Ag/AgCl | [38] |
Principal preparation parameters and results reported for epoxy-silica coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density, Icorr, and corrosion potential, Ecorr.
GPTMS: (3-glycidoxypropyl)trimethoxysilane; TEOS: tetraethoxysilane; APTES: aminopropyl-triethoxysilane; tetrathiol: pentaerythritol tetrakis(3-mercaptopropionate); MTEOS: methyl-triethoxysilane; SCE: standard calomel electrode.
Microscratch and wear tests, performed with a spherical-conical diamond tip of 10 μm radius, confirmed for the PMMA-silica coatings that both additives, CNT and GO, improved the scratch resistance (increase of the friction coefficient by 0.1−0.2), adhesion to the metallic substrate (no delamination for M8_BPO001_CNT up to 240 mN) and wear resistance (smooth and shallow wear track after 50 cycles). The superior behavior of CNT containing coatings was attributed to their property to act as rigid obstacles for the scratch tip. Results of the thermogravimetric analysis have shown that the addition of CNT and GO to the BPO0.01 matrix and to a smaller extent to the BPO0.05 matrix, increased the thermal stability of the hybrids up to 70°C for GO containing samples (Table 6). This improvement was attributed to interaction between carbon nanostructures and macroradicals generated during the process of depolymerisation combined with a 2D barrier effect of GO, hindering molecular diffusion through the matrix and thus providing an improved thermal resistance [18].
Results of electrochemical impedance spectroscopy in 3.5% NaCl solution showed that PMMA-silica coatings reinforced with CNTs and GO had an improved anticorrosive efficiency, with impedance modulus of ~1 GΩ cm2 and ~10 GΩ cm2 for the BPO0.01 and BPO0.05 matrix, respectively. Besides the improved barrier property, GO containing coatings presented also a prolonged lifetime of up to 203 days. This was attributed to the two dimensionality of the GO structure that provides an enhanced barrier effect against the propagation of corrosive species. Furthermore, it was suggested that both carbon nanostructures act as densifiers of the nanocomposite and also as negatively charged repulsive agents for chloride anions, thus improving barrier property of the coating. Based on the equivalent circuit used to fit the EIS data, this notable barrier behavior was interpreted, in terms of two distinct dielectric layers, one related to a porous water uptake zone at the coating/electrolyte interface and the other corresponding to the underlying unaffected film region, having three orders of magnitude higher resistivity [18].
To be able to evaluate the relevance of the obtained results, it is important to place them in the context of the state of the art in the field of anticorrosive coatings. In the last decade, the concept of organic-inorganic hybrids as protective coating has been intensely investigated using different approaches involving a variety of organic and inorganic precursor reagents, resulting in a number of promising coatings systems. The most widely applied formulations for hybrid phases used to prepare high-performance anticorrosive coatings are based on epoxy-silica (Table 7) and acrylic-silica (Table 8) hybrids, and to a lower extent on polyurethane-silica, polyurethane-silica-zirconia and other epoxy systems (Table 9). As can be inferred from these data, the electrochemical barrier properties, obtained for different hybrid formulations, have achieved a notable performance in the last years, making these novel nanocomposites very promising candidates for efficient corrosion protection of metallic surfaces. This is justified especially when considering that a high-corrosion resistance and long durability in aggressive environments can be achieved by thin films with thicknesses of less than 10 μm, resulting in substantially reduced material costs compared to conventional high-performance coating systems. More specifically, regarding the epoxy-silica hybrid system, the results presented in this work and those listed in Table 7 show that different compositions applied to distinct alloys can provide a very effective long-term corrosion protection [7, 26, 27]. Very promising results were also achieved for PMMA-silica coatings [8, 10, 31], with the highest observed durability of more than 560 days in 3.5% NaCl, and for hybrids containing reinforcement and inhibitor agents [11, 18]. Moreover, for some polyurethane-silica and polyurethane-zirconia-silica systems, it has been demonstrated that they also have a high potential to be used as efficient anticorrosive barrier layers [9, 32].
Hybrid/additive | Synthesis | Substrate | Deposition/thickness (μm) | EIS: |Z| (GΩ cm2), lifetime (days), solution | PolarizationIcorr (A cm−2) Ecorr (V), reference electrode | Reference |
---|---|---|---|---|---|---|
PMMA-MPTS-TEOS/- | Radical polymerization and sol-gel | Al alloy AA2024 | Dip-coating/~3 | ~50 more than 560 3.5% NaCl | _ −0.68 Ag/AgCl | [This work] |
PMMA-MPTS-TEOS/- | Sol-gel | 316L stainless steel | Dip-coating/~2 | ~0.01 36 3.5% NaCl | 10−9 0.1 Ag/AgCl | [14] |
GMA-EHA-GPTMS-TEOS/- | Sol-gel | Al alloys AA1050 | Spin-coating/~1 | ~1 21 0.1 M NaCl | _ | [39] |
PMMA-MPTS-TEOS/- | Sol-gel | A1010 carbon steel | Dip-coating/1.5–3 | ~1 18 3.5% NaCl | 10−10 −0.3 Ag/AgCl | [22] |
PMMA-MPTS-TEOS/Ce(IV) | Radical polymerization and sol-gel | A1010 carbon steel | Dip-coating/~2 | ~10 304/404 3.5% NaCl | 10−11 + 0.3 Ag/AgCl | [11] |
PMMA-MPTS-TEOS/ lignin | Radical polymerization and sol-gel | A1020 carbon steel | Dip-coating/~2 | ~0.5 50 3.5% NaCl | _ | [20] |
PMMA-MPTS-TEOS/- | Radical polymerization and sol-gel | A1010 carbon steel | Dip-coating/1.5–2 | ~5 196 3.5% NaCl | _ | [10] |
PMMA-MPTSTEOS/Ce | Sol-gel | Mild steel | Dip-coating/~26 | ~10 362 3.5% NaCl | 10−12 + 0.35 SCE | [31] |
Acrylic resin-SiO2/- | Solution intercalation method | Mild steel | Brush method/75 | ~10 90 3.5% NaCl | _ | [8] |
Acrylic resin-silanol-ZnO/- | Solution intercalation method | Mild steel | Brush method/75 | ~10 30 3.5% NaCl | _ | [40] |
PMMA-MPTS-TEOS/CNTs, GO | Radical polymerization and sol-gel | A1020 carbon steel | Dip-coating/3–6 | ~3 211 3.5% NaCl | _ +0.58 Ag/AgCl | [18] |
Principal preparation parameters and results reported for acrylic-silica coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density Icorr, and corrosion potential Ecorr.
PMMA: poly(methyl methacrylate); MPTS: 3-(trimethoxysilyl)propyl methacrylate; TEOS: tetraethoxysilane; GMA: glycidyl methacrylate; EHA: 2-ethylhexyl acrylate; GPTMS: (3-glycidoxypropyl) trimethoxysilane;CNTs: carbon nanotubes; GO:graphene oxide; SCE: standard calomel electrode.
Hybrid/additive | Synthesis | Substrate | Deposition/thickness (μm) | EIS: |Z| (GΩ cm2), lifetime (days), solution | Polarization,Icorr(A cm−2),Ecorr (V),reference electrode | Reference |
---|---|---|---|---|---|---|
Polyurethane APTES-TEOS/- | _ | AA3003 H14 | Drawdown bar/75 | ~100 263 3.5%NaCl+0.1M HCl | _ | [9] |
Polyurethane-ZrO2-SiO2/- | Sol-gel | Carbon steel | Spray/40–55 | ~100 226 3.5% NaCl | _ | [32] |
Epoxy-polyaniline-ZnO/- | Chemical oxidative method | Carbon steel | Dip-coating/118 | ~1 90 3.5% NaCl | _ −0.05 (SCE) | [41] |
Polyetherimide-HA/- | _ | Mg alloy AZ31 | Dip-coating/~4 | ~1 101 Hank’s solution | _ | [42] |
Epoxy-LDH/- | _ | Al alloy AA2024-T3 | Spray/55 | ~1 18 0.05M NaCl | _ | [43] |
Epoxy-HS/8-hydroyquinoline | _ | Al alloy AA2024-T3 | Dip-coating/~25 | ~1 90 0.5M NaCl | _ | [44] |
Epoxy-CaCO3 | _ | Al alloy AA2024-T3 | Dip-coating/~30 | ~1 41 0.5M NaCl | _ | [45] |
Epoxy-ester-siloxane-urea | _ | Al alloy AA2024-T3 | Drop-coating/130–140 | ~0.1 70 3.5% NaCl | 10−10 −0.4 SCE | [46] |
Principal preparation parameters and results reported for a varied of hybrid coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density Icorr, and corrosion potential Ecorr.
APTES: aminopropyltriethoxysilane; TEOS: tetraethoxysilane; HA: hydroxyapatite; LDH: Layered double hydroxide; HS: halloysites; CaCO3: calcium carbonate; SCE: standard calomel electrode.
All these results demonstrate the wealth of possibilities to prepare nanocomposite materials based on organic-inorganic hybrids in the form of highly efficient anticorrosive coatings. The optimization of the barrier property can be achieved by the careful adjustment of the precursor proportions, including coupling agents and additives, together with the conditions of synthesis, deposition, and thermal treatment. However, the main key for this task is an in-depth knowledge of the formation mechanisms as well as the compositional and structural properties of the material. Based on this information, it was shown that a relatively simple preparation process yields highly efficient and very durable anticorrosive films. They unite three essential perquisites for an appropriate coating system: a high corrosion resistance, long-term stability, and environmental compatibility. Considering also the simplicity of the sol-gel process and the low material consumption, which scales with the film thickness, these about 5-μm thick hybrid films constitute from the economical and environmental point of view a very interesting alternative for conventional protective coating systems.
Structural, thermal, mechanical, and electrochemical characterization of novel epoxy-silica and PMMA-silica hybrid coatings have shown that their properties are extremely dependent of the hybrid precursors proportion, time and temperature of synthesis, and addition of fillers. After a careful adjustment of the preparation conditions, these homogeneous and transparent hybrid coatings present a defect-free very smooth surface, low porosity, a highly cross-linked silica network, excellent adhesion to the metallic substrate, elevated thermal stability, and especially an excellent anticorrosive performance. Epoxy-silica and PMMA-silica films with thicknesses of less than 10 μm exhibit a dense and highly reticulated nanostructure, resulting in enhanced thermal stability combined with high corrosion resistance and long durability in saline environment. Exceptional barrier properties, especially on aluminum alloy, were found for PMMA-silica hybrids prepared at a 8MMA:1MPTS:2TEOS molar ratio, 4 h/80°C of synthesis, and BPO/MMA molar ratio of 0.01. This coating highlights a corrosion resistance of about 50 GΩ cm2 and a lifetime of more than 18 months in saline solution. Nanofillers have been successfully added to the PMMA-silica matrix to improve the anticorrosive performance and to reinforce the hybrid structure. Carbon nanotubes and graphene oxide incorporated into the PMMA-silica matrix resulted in a multifunctional material, which combines an excellent anticorrosive performance with improved adhesion, anti-scratch and heat-resistant properties, thus extending the application range of these coatings to abrasive environments. Furthermore, it has been shown that added Ce(IV) ions act as oxidation agents during the formation of the hybrid matrix, leading to densification process that improves the barrier property of the coatings. In addition, the active corrosion inhibition provided by formation of insoluble cerium species in regions affected by corrosion, known as self-healing ability, resulted in a prolonged the lifetime of the coatings. The great progress achieved in the last couple of years in the development of organic-inorganic hybrids makes these materials very promising candidates for new-generation high-performance protective coatings.
We would like to thank the National Laboratory of Synchrotron Light Source (LNLS, Brazil) for the use of SAXS facilities. This work was supported by the Brazilian funding agencies CNPq, CAPES, and FAPESP.
Food emulsions are produced from two immiscible liquids (usually oil and water), which in the presence of an emulsifier and by applying an emulsification method, can be dispersed one into another. Some typical examples include mayonnaise, salad dressings, sauces, milk, ice cream, and sausages. These systems can be used to encapsulate, protect, and deliver biocompounds, including vitamins, flavors, colorants, and nutraceuticals [1]. Emulsifiers are food additives acting by forming a physical barrier between the oil and water, enabling their compatibilization. Effective emulsifiers must be quickly adsorbed at the oil–water interfaces leading to a rapid decrease in the interfacial tension, preventing droplets aggregation. Moreover, they must generate strong repulsive interactions promoting emulsion stability [2, 3].
Synthetic emulsifiers (e.g., Tweens and Spans) are well-known for their ability to form highly stable emulsions. However, consumers’ preferences for healthy, sustainable and natural lifestyle habits have increased worldwide. Moreover, some studies have reported intestinal dysfunctions caused by synthetic emulsifiers [4, 5]. In this context, natural emulsifiers have emerged as great alternatives to replace their conventional counterparts, namely proteins [6], polysaccharides [7], phospholipids [8] and saponins [9]. Concerning protein-based natural emulsifiers, the most use ones come from animal sources (e.g., whey proteins, caseins, egg protein, gelatin) [10]. However, plant-based proteins have demonstrated to be good alternatives for their replacement in products with dietary restrictions (e.g., lactose-free) and in vegetarian and vegan foods. Moreover, plant-based proteins are more sustainable as they have a lower carbon footprint [11, 12]. Examples include pea [13, 14] and soy proteins [15], which have been reported for emulsions production.
Aligned with natural emulsifiers, Pickering stabilizers (in particular organic-based colloidal particles) are emerging as promising solutions. Pickering emulsions or particle-stabilized emulsions present high resistance to coalescence and Oswald ripening due to the tight fixation of the particles to the droplets surface [16]. Several food-grade particles have been studied, namely particles based on proteins [17], polysaccharides [18], and protein/polysaccharide complexes [19]. Furthermore, natural emulsifiers from microbial origin such as biosurfactants and bioemulsifiers are also potential alternatives to be explored in food emulsions [20, 21].
This chapter covers a bibliographic review focused on the last 10-years on natural emulsifiers and emulsion technology field. Research and market trends are also highlighted, showing the most relevant natural emulsifier families. Basic concepts concerning emulsion production, classification, and stabilization methods are introduced. A special emphasis is given to Pickering emulsions regarding novel trends in food emulsion systems.
According to the Research and Markets report, amidst the Covid-19 crisis, the global emulsifiers’ market is projected to reach US$ 6.1 Billion by 2027, growing at a Compound Annual Growth Rate (CAGR) of 4.8% over the forecast period (2020–2027). Particularly, natural emulsifiers’ area is estimated to get US$ 3.3 Billion, recording a 5.4% CAGR [22]. In agreement, the “Global Food Emulsifiers Market 2020-2027 report” from MarketResearch, foresees a high potential for the plant-based emulsifiers in the global food emulsifiers market [23].
Concurrently, scientific literature corroborates the global food emulsifiers report’s projections. More than 8,000 documents were found using the terms “natural emulsifier*” OR “bioemulsifier*” OR “bio-emulsifier*” OR “biosurfactant*” OR “bio-surfactant*” OR “Pickering emulsion*” searched in title, abstract, keywords and Keywords plus sections using the Web of Science Core Collection (SCI-EXPANDED), in the 2010–2020 period. Excluding documents with early publication and applying the “Food Science and Technology” filter from WOS, 792 documents were found. By removing 4 documents from 2021 in a final manual screening, 788 documents were analyzed using Biblioshiny app from the Bibliometrix-R package (RStudio) [24] and VosViewer software [25]. The survey was performed on April 25th, 2021.
Table 1 presents some of the retrieved 788 documents concerning the application of natural emulsifiers or Pickering stabilizers in emulsion formation/stability, including their use in biocompound delivery systems. Some works regarding the production of bioemulsifiers or biosurfactants by microorganisms were also found [31, 32]. Several studies addressing Pickering emulsions and the use of high-pressure homogenization were identified.
Natural emulsifiers or Pickering stabilizers | Emulsification method | Main target | Reference |
---|---|---|---|
Zein-Chitosan complex particles | High-shear homogenization | Delivery system (Curcumin) | [26] |
Mannoprotein | High-shear homogenization | Formation/Stability | [27] |
Whey protein Gum arabic Lecithin | Dual-channel microfluidization | Formation/Stability | [28] |
Gum arabic Beet pectin Corn fiber gum | High-shear homogenization Microfluidizer | Formation/Stability | [7] |
Ginseng saponins | High-shear/ High-pressure homogenization | Delivery system (Astaxanthin) | [9] |
Wheat gluten nanoparticles (WPN) WPN-xanthan gum nanoparticles | High-shear homogenization | Delivery system (β-carotene) | [29] |
Pea protein microgel particles | High-pressure homogenization | Formation/Stability | [30] |
Studies reporting the use of natural molecules and Pickering stabilizers selected from the retrieved 788 documents of the bibliometric search.
Figure 1a shows the wordcloud from Author’s Keyword. The higher font size indicates an increased frequency of the keyword. Figure 1b also illustrates keyword co-occurrence network analysis; the terms distributed in the same cluster present the higher similarity, in comparison with the terms distributed in different clusters.
(a) Wordcloud from Author’s keywords (100 keywords; minimum frequency of 5); (b) keyword co-occurrence network (9 clusters; Author’s keywords; number of occurrences 5).
“Pickering emulsions” is the most frequent keyword, followed by biosurfactant (Figure 1). Other keywords (e.g., whey protein, sodium caseinate, glycolipid, sophorolipids, rhamnolipids,
These findings substantiate the keyword co-occurrence analysis (Figure 1). 93 keywords (Author’s keywords) were organized in 9 clusters. The number of occurrences indicates the number of documents where the keyword appears. Each circle represents a keyword with at least 5 occurrences, being their areas proportional to the number of occurrences. The clusters are characterized by different colors and their words can be related.
Some clusters present words associated to recent trends in the area of natural emulsifiers. Clusters 1, 6, 8 and 9 refer to “Pickering emulsions” and other inter-related words, including nanoparticles, Pickering stabilization, and some commonly used Pickering stabilizers such as starch granules, cellulose nanocrystals and kafirin. Clusters 1 and 2 comprise terms related to the rheological properties of emulsions, an important parameter in food applications. The words included in clusters 4 and 5 are associated with microorganisms (e.g.,
Cluster 7 and 9 are centered in words related to the biocompounds delivery systems, namely bioavailability/bioaccessibility, controlled release, encapsulation and examples of used biocompounds, such as beta-carotene, curcumin, and vitamin E. Clusters 8 and 9 refer to proteins, phospholipids, saponins and polysaccharides, such as whey protein isolate, soy lecithin,
In a general overview, the analysis showed the progressive interest in natural emulsifiers due to their relevance for the scientific and industrial communities, as well as for the global market. Moreover, Pickering emulsions are emerging as advanced emulsion technologies within future trends in the food industry.
Natural emulsifiers belong to a broad range of chemical families and some main examples are shown in Figure 2. Within each family, aspects such as the used natural source or extraction method can lead to different properties. Therefore, the next sections summarize the most relevant families in the area of natural emulsifiers and their contextualization in the field of food applications.
Representative chemical structures for each emulsifier family.
Phospholipids are amphiphilic molecules, and a main constituent of natural membranes. Their structure comprises a hydrophilic head holding a phosphoric acid (H3PO4), combined with a hydrophobic tail composed by one or two non-polar fatty acids. They comprise groups as glycerophospholipids or sphingolipids, with lecithins (glycerophospholipid) assuming an important role. Phospholipids can be obtained from diverse natural sources, including milk, vegetable oils (soybean, rapeseed or sunflower), egg yolk, meat and fish [36, 37]. Specifically, lecithins are known to be good stabilizers for food emulsions, for example the ones derived from soy or egg yolk are applied in mayonnaise, creams, or sauces [38]. Other phospholipid examples include phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, phosphatidic acid, sphingomyelin. The amphiphilic character of these compounds supports their capacity to stabilize emulsions. Concurrently to their ability to stabilize emulsions they can act as texturizing agents, thus influencing the organoleptic attributes of the final product [39].
Saponins are a complex family derived from plants, constituted by triterpenes or steroid aglycones linked to glycosyl derived sugar structures [40]. Usually the aglycones involve pentacyclic triterpenoids with oleanolic acid and the sugars moieties comprise rhamnose, xylose, glucose or galactose [41]. Factors conditioning the composition of saponins are their botanical origin and extraction method.
Proteins are molecules resulting from the combination of 21 different amino acids, having diverse properties, including water solubility, which varies depending on their composition [46]. Structurally, the presence of both hydrophobic and hydrophilic amino acids confer an amphiphilic character, allowing them to be absorbed at oil/water interfaces, leading to emulsion stabilization [47]. However, proteins have low surface activity in comparison with conventional emulsifiers. This is attributed to the random distribution of the hydrophilic and hydrophobic groups within the peptide chains, limiting their adsorption. This effect is balanced by the protein film formation around the droplets, leading to stabilization through molecular interactions [48]. Diverse proteins (e.g., whey, casein, soy or faba bean proteins) have been tested in food applications, e.g., emulsions for the controlled release of lutein [49], ѡ-3 oil [50], bioactive hydrophobic compounds [51], fish oil [52, 53] or β-carotene [54]. Their application in final products is still hindered by environmental conditions such as pH, temperature and ionic strength [48]. However, these drawbacks can be surpassed by using more complex formulations, namely by combining proteins with polysaccharides [48] or by chemically modifying the proteins trough grafting with other compounds such as polyphenols [54].
Polysaccharides are biopolymers composed of monosaccharide units such as glucose, fructose, mannose or galactose, bonded by glycosidic bonds. Their structural rearrangement, i.e., type and number of monosaccharides, type of glycosidic bonds, molecular weight, electrical charge, branching degree, hydrophobicity and the presence of other groups (carboxylate, sulfate or phosphate), rule the polysaccharides functional properties such as solubility, rheology, and amphiphilic character, among others [10]. Their amphiphilicity depends on the presence of hydrophobic (glycolipids) and hydrophilic (hydroxyls) groups, being adsorbed at the interface, forming a thick stabilizing layer (e.g., pectins, gum Arabic) [55]. Moreover, non-amphiphilic polysaccharides can contribute to emulsion stabilization due to their thickener role, increasing the viscosity and decreasing oil droplets’ motion (e.g., alginates, carrageenan) [56]. Despite the high number of polysaccharides available in nature, only few are authorized as food emulsifiers in EU, namely alginic acid (E400), gum Arabic (E414), pectin (E440), cellulose and chemically modified celluloses (E460 to E469) [57]. Polysaccharides can be obtained from animal, vegetal, microbial fermentation or marine sources (algae), being their properties mostly dependent on the source and extraction process [10].
Microbial synthetic routes are emerging as valuable sustainable and green alternatives to produce emulsifiers. They generate compounds with low ecotoxicity, biodegradability, stability (pH and salinity) and low critical micellar concentration (CMC), in addition to biological activity, biocompatibility and digestibility [58]. Emulsifiers produced by microorganisms are classified according to their molecular weight. Low molecular weight family includes glycolipids (e.g., rhamnolipids, sophorolipids, trehalose lipids) and lipopeptides (e.g., surfactin, iturin, fengycin) and are referred as biosurfactants. Polysaccharides, proteins, lipoproteins, and lipopolysaccharides belong to the high molecular weight family and are referred as bioemulsifiers [59, 60]. Glycolipids like rhamnolipids and trehalose lipids are mostly produced by bacterial strains like
Emulsions are colloidal systems constituted by two immiscible liquids (oil and water), formed in the presence of an emulsifier, and, usually, by applying an energy input. The emulsifier selection is therefore an important step to reach stability. They can be classified based on the hydrophilic region that correspond to ionic structures (anionic or cationic surfactants), change charge with pH (amphoteric surfactants) or present no charged centers (nonionic surfactants) [69]. Among them, nonionic surfactants are often used in food applications because they are less toxic and less affected by pH and ionic strength changes [70, 71]. The choice of a nonionic surfactant can be based on the hydrophilic–lipophilic balance (HLB) index [72]. This scale (0–20), reflects the changing from hydrophobic to hydrophilic character, that is, a lower HLB value corresponds to a lipophilic surfactant being appropriate to stabilize water-in-oil (W/O) emulsions, whereas a high HLB indicates the ability to stabilize oil-in-water (O/W) emulsions, due to the strong hydrophilic balance [72].
Emulsions can be classified according to their typology and structure. The first refers to the relative distribution of the immiscible phases (oil and water), and the latter refers to the arrangement of the emulsified entities [73]. Considering the typology, they can be classified as simple (O/W and W/O) or double (oil-in-water-in-oil (O/W/O), and water-in-oil-in-water (W/O/W)) emulsions (Figure 3). Examples of O/W emulsions in food systems include products such as milk, sauces, beverages, yogurts, ice-creams, and mayonnaise [74]. W/O emulsions are not so frequent but can be found in butter and margarine [73, 75]. For double emulsions, W/O/W are the most used systems due to their ability to generate reduced-fat products, when compared to O/W emulsions. Moreover, they can serve as base systems to encapsulate and control the release of sensitive water-soluble compounds, such as flavors or bioactive ingredients [16, 75, 76].
Typology of simple and double emulsions.
Regarding structure, emulsions can be classified as macroemulsions (usually called emulsions), nanoemulsions, or microemulsions. These systems present specific physicochemical properties that influence their range of applications [71]. Emulsions and nanoemulsions are thermodynamically unstable systems because their free energy is higher than the one of the individual phases [74, 77]. Thus, considering that all systems tend to their lowest energy state, phase separation will occur. However, due to their kinetic stability, they may remain in a metastable state for a considerable period of time, delaying the phase separation phenomenon. The kinetic stability is governed by two mechanisms, namely the energy barriers between the two states (emulsified and separated phases) and mass transfer between the phases. Therefore, high energy barriers and slow mass transfer processes delay phase separation [78]. By contrast, microemulsions are thermodynamically stable systems because their free energy is lower than the one of separate phases. Thus, they can be formed spontaneously under particular compositions and temperature conditions. In practice, some energy input is needed due to the existence of kinetic energy barriers [71]. Regarding the droplet size, nanoemulsions and microemulsions present droplet sizes <200 nm, whereas emulsions hold sizes between 200 nm and 100 μm [16, 71].
Nanoemulsions and microemulsions are optically transparent or slightly turbid due to their small droplet size, being valuable for applications requiring transparency, such as soft drinks [79]. Comparatively with nanoemulsions, microemulsions require a higher emulsifier content, have a lower particle size, and droplets can assume a non-spherical shape, feature that can be used to differentiate the two systems. Emulsions are typically turbid to opaque and are used in creamy systems such as dairy products [80]. Table 2 provides some application examples for each system addressing natural emulsifiers.
System | Type | Natural emulsifiers | Final applications | Reference |
---|---|---|---|---|
Emulsions | O/W | Whey protein | Ice-cream | [81] |
O/W | Coffee creamers | [82] | ||
W/O/W | Whey, rice and pumpkin seed proteins | Cheese | [83] | |
O/W | Faba bean protein | Tofu and yogurts | [84] | |
O/W | Pectin | Functional foods | [85] | |
O/W | Rhamnolipid | Beverages | [86] | |
O/W | Whey protein, locust bean gum, and iota-carrageenan | Mayonnaise, salad dressings, and sauces | [87] | |
O/W | Modified starch | Dairy products | [88] | |
Nanoemulsions | O/W | Soy lecithin | Fruit juices | [89] |
O/W | Soft foods, creams, sauces, and salad dressings | [90] | ||
O/W | Functional foods and beverages | [91] | ||
O/W | Ginseng saponin | [9] | ||
O/W | Whey protein, gum Arabic, and soy lecithin | [92] | ||
O/W | Modified starch, whey and casein proteins | [93] | ||
Microemulsions | W/O | Soy lecithin | Extraction of edible vegetable oils | [94] |
W/O | Soy lecithin | Functional foods and soft drinks | [95] | |
O/W | Soy lecithin | [96] | ||
O/W | Soft drinks and minced meat | [97] |
Food applications of emulsions, nanoemulsions and microemulsions using natural emulsifiers.
Emulsions are thermodynamically unstable mixtures, characterized by the presence of at least two immiscible phases and an emulsifier that, when provided with enough mixing energy, are able to maintain stability over time [98]. The role of the emulsifier is essential to assure stable long-term properties. In general, emulsifiers are active surface substances, enabling their positioning at the oil–water interface, reducing the interfacial tension, hindering (or delaying) aggregation phenomena [99]. Typically, the hydrophilic part of the emulsifier is located in the aqueous phase, while the hydrophobic tail remains enclosed in the oil phase [82, 100]. During emulsion formation, the surfactant molecules require time to move to the interface, forming a layer to reach the interfacial tension equilibrium, a phenomenon related with their adsorption kinetics [82]. This pattern is dependent on emulsifiers’ nature, taking from minutes (e.g., some saponins) to hours (e.g., some proteins), besides being dependent on environmental conditions (e.g., pH, temperature) [82]. To note that, even emulsions are commonly stabilized by a monolayer structure around the droplets, multilayer structures can also be formed. The multilayer pattern favors the electrostatic and steric repulsion of the droplets, improving stability while providing additional protection to the internal phase [16].
The emulsion stabilization mechanism can differ depending on the nature of the used surfactant. In this context four principal stabilization mechanisms are known, namely electrostatic repulsion, steric repulsion, Marangoni-Gibbs effect, and thin film stabilization mechanisms [101]. The electrostatic repulsion is related to ionic emulsifiers and consists on the formation of an electrical double layer at the droplet’s interface, hindering their approximation. Steric repulsion is characteristic of nonionic and/or polymeric emulsifiers, and droplet’s distance is kept due to the adsorption of the hydrophobic segment by the oil phase [101]. The Marangoni-Gibbs effect preserve emulsions’ structure through the deformation of adjacent droplet’s surface, avoiding their outflow, whereas the thin film stabilization mechanism avail the stability of the emulsion by generating a rigid and viscoelastic film, preventing droplets from destabilization effects [101].
Other factors can condition emulsion’s stabilization mechanism, including the emulsifier content, the oil to water ratio or the preparation conditions (pH or temperature). For example, some phospholipids can have no charge at neutral pH, turning into anionic at acidic media, promoting molecule’s swelling at the interface [100]. Moreover, the surfactant concentration can have also impact, e.g., sunflower lecithin in O/W emulsions, at low contents, create a layer surrounding the oil droplets, while at higher concentrations, the stabilization mechanism changes, producing, concurrently, liposomes that might destabilize the emulsion [10]. Considering amphiphilic polymers, when they are used as emulsifiers, they become positioned at the interface, just like the small molecules, but their ability to create intermolecular interactions can provide additional stabilization effects. Their effect on viscosity can also provide a positive stabilization effect [102]. The high hydrophilicity of most polysaccharides can difficult their emulsifier role, if considering the importance of the emulsifiers’ hydrophilic/lipophilic character to interact with both phases. This constraint can be overcome by either chemical or physical strategies [103]. Namely, the suitability of anchoring hydrophobic groups into the polysaccharide structure can equilibrate the hydrophilic/lipophilic balance, that is the hydrophobisation of emulsifier’s surface. Otherwise, alternative approaches imply the mixture of the polysaccharides with other polymers (co-surfactants) to favor the hydrophilic/lipophilic equilibrium and stabilization role.
Food emulsions can be produced using several methods, classified as low-energy and high-energy processes, as represented schematically in Figure 4. The selection of the most appropriate method and respective equipment is based on the volume to process, characteristics of the initial mixture, emulsion’s physicochemical properties, droplet size, and process costs [104]. In Table 3 a survey of recent works dealing with emulsion production trough different methods and using natural emulsifiers in their pure form or compounded with synthetic emulsifiers is presented. Moreover, their potential to encapsulate bioactives for food industry applications is also described.
Schematic representation of the emulsification process through high- and low-energy methodologies.
Productive method | Emulsion type | Oil phase | Particle diameter | Bioactive compound | Emulsifiers | Reference |
---|---|---|---|---|---|---|
Spontaneous emulsification | O/W | MCT | > 10 μm | — | Sunflower phospholipids | [36] |
Emulsion phase inversion | W/O | MCT and orange oil | > 10 μm | Vitamin E | WPI; SMP; Casein; | [105] |
Phase inversion temperature | O/W | Peppermint oil | < 12 nm | Coenzyme Q10 | Lecithin/Tween 20 | [106] |
High-pressure homogenization | O/W | Paprika oleoresin | <150 nm | — | Soy lecithin; WPC; Gum Arabic | [92] |
Microfluidization | O/W | Fish oil | <150 nm | Omega-3 fatty acids | Sunflower phospholipids | [8] |
Colloid mills | W/O | Rapeseed oil | 4.8 μm | — | WPI; Pectin | [107] |
Ultrasonic homogenizer | O/W | MCT; Palm oil; Soybean oil; Rapeseed oil | 0.5–24.1 μm | — | Soy protein isolate | [6] |
Membrane emulsification | W/O/W | Sunflower oil | 35–320 μm | Magnesium | Starch; Pea protein isolate | [108] |
Microchannel homogenizer | O/W | Soybean oil | 35–47 μm | — | Sodium alginate; Pectin; Gum Arabic; Carboxymethyl cellulose | [109] |
High-speed homogenization | O/W | Soybean oil | 143.5 nm | β-carotene; Eugenol | Lecithin; WPI | [110] |
Studies applying different productive methods using natural emulsifiers or natural/synthetic blends to form emulsions and/or to encapsulate biocompounds for food industry applications.
WPI: Whey protein isolate; WPC: Whey protein concentrate; MCT: medium-chain triglycerides; SMP: Sucrose monopalmitate.
Low-energy methods comprise, spontaneous emulsion, and emulsion phase inversion (e.g., phase inversion composition and phase inversion temperature), which occur due to environmental or composition changes namely temperature, pH, and ionic strength of the formulation [104]. Low-energy approaches are more cost effective than high-energy methods. However, they are limited to certain oils and emulsifiers, requiring also large amounts of surfactants, which is not desirable for many food applications [71]. In the work reported by Komaiko et al. [36], spontaneous emulsification lead to emulsions with large droplet size (>10 μm), comparatively with those produced by high-energy methods (<10 μm). The authors concluded that natural emulsifiers can be used in SE emulsions for applications where fine droplets are not essential (Table 3). By contrast, Mayer et al. [105] concluded that it was not possible to produce nanoemulsions using natural emulsifiers by the emulsion phase inversion method. These limitations imply that even natural-based emulsions can be prepared through low-energy methods, high-energy approaches are needed when natural emulsifiers are used.
High-energy methods generate intensive forces promoting the water and oil phases disruption and their subsequent mixture. High-shear homogenizers are the most used equipment’s for producing emulsions in the food industry. They consist on a rotor-stator or stirrer device able to mix the components at high speeds. Usually, large droplets are produced using this approach (1–10 μm) in comparison to alternative high-energy methods. High-pressure homogenization is also widely used in the food industry, being more effective to reduce the droplet size of a pre-emulsion. Generally, this coarse pre-emulsion is produced by high-shear homogenizers, then subjected to the high-pressure homogenization process. The equipment consists of a high-pressure pump (3–500 MPa) to pass the coarse emulsion through a narrow homogenizing valve, generating intensive disruptive forces (shear and cavitation), breaking down the droplets into smaller ones [80, 81].
Many studies have been conducted using two high-energy sequential methods (high-shear and -pressure homogenizers) to produce emulsions/nanoemulsions with natural emulsifiers [111, 112, 113]. Flores-Andrade and co-workers performed a study with soy lecithin, whey protein concentrate (WPC) and gum Arabic as natural emulsifiers, and paprika oleoresin as the oil phase. The coarse emulsion was produced by a high-speed homogenizer, then treated in a high-pressure homogenizer. O/W nanoemulsions were produced, being WPC more effective to form small droplets (d < 150 nm) than the other tested emulsifiers [92].
Microfluidization is the most effective method for producing emulsions with fine droplets (d < 100 nm). This approach is based on feeding the fluid into the homogenizer, which consists of a mixture chamber with two channels. Intensive disruptive forces are generated when these two fluid streams collide at high speed, breaking larger droplets and intermingling the fluids [3]. As the high-pressure homogenizers, microfluidizers were used after preparing a pre-emulsion by high-shear mixers [42, 114]. Ultrasound technique uses high-intense ultrasonic waves, generating intense shear and pressure gradients. The droplets are disrupted mainly by cavitation and turbulent effects [99, 115].
Currently, high-energy approaches are commonly used in the food industry due to their large-scale production capacity and the possibility to process a wide range of raw ingredients [71]. Although several high-energy emulsification devices are available, high-shear and pressure homogenizers, microfluidizers and ultrasound equipment’s are the most used in the production of natural emulsifiers-based emulsions.
Emulsion stability is an important parameter indicating its ability to resist physicochemical changes over time [116]. For food emulsions, the required stability varies according to the intended final application. For example, short-term stability of minutes to hours, is enough for intermediate food emulsions such as cake batter and ice cream mixtures, while long-term stability is required for long shelf-life products, including mayonnaise and salad dressings [117]. For the latter ones, the development of effective strategies to retard emulsion destabilization implies the identification of the main mechanisms leading to this effect [73].
Emulsion instability can occur due to physical and/or chemical processes. The physical instability is responsible for modifying the emulsion droplets spatial distribution and structure, including gravitational separation (creaming/sedimentation), flocculation, coalescence, and Ostwald ripening phenomena (Figure 5). These effects depend on the emulsion composition and structure, besides the storage conditions, namely temperature variation and mechanical stirring [74, 116]. Moreover, the physical phenomena are interrelated and can influence each other during emulsion storage [77].
Common types of instability phenomena in emulsions.
Gravitational separation is driven by density differences between the droplets and the continuous phase. The droplets are subjected to gravitational forces tending to accumulate in the top (creaming) or in the bottom (sedimentation) of the system. Most edible oils present densities lower than water, favoring creaming in O/W emulsions, whereas sedimentation is usually observed in W/O emulsions [116]. Considering the impact of gravitational forces in the large droplets, the separation usually occurs for emulsions with droplet sizes higher than 100 nm or in a final stage of a sequence of instability phenomena [116]. By contrast, for lower droplet sizes, e.g., nanoemulsions, Brownian motion dominates over gravitational forces. Thus, reducing the droplet size is a suitable strategy to retard gravitational separation, with the emulsifier playing an important role to effectively reduce droplets’ size [2, 74]. Furthermore, the emulsifier’ layers tend to minimize the density difference between the emulsion phases, thus reducing the velocity of gravitational separation. Other strategies include modifying the rheology of the continuous phase or increasing the concentration of the droplets [74, 116].
Ostwald Ripening consists of the increase of the droplets size due to the diffusion of small droplets into larger ones, effect driven by their solubility in the continuous phase. This effect is promoted when the droplet’s size decreases [73], being also influenced by the emulsifiers’ properties. Namely, Ostwald Ripening can be retarded by decreasing the interfacial tension of the phases, favored when small-molecule surfactants are used or when using emulsifiers able to form rigid shell around the droplets. By contrast, emulsifiers prone to solubilize the oil and water phases through the formation of colloidal structures (e.g., micelles) accelerate the Ostwald Ripening [2].
Flocculation and coalescence mechanisms are related to droplets aggregation, effect leading to droplet size increase [74]. In flocculation the association of at least two droplets in an aggregate occurs, whereas in the coalescence, the droplets merge into a larger one [77]. Both phenomena are highly dependent on the selected emulsifier [77, 116], namely their nature and colloidal interactions’ capacity [2].
Pickering emulsions are defined as systems stabilized by solid colloidal particles adsorbed at the oil–water interface in a practically irreversible process, creating a coating around the droplets, either in the form of a single or multiple layer, generating a strong steric barrier providing high stability [118]. In the context of Pickering emulsions, the search for natural-based particles is currently a hot topic to face market demands for novel clean label products (absent of emulsifiers) [119]. Pickering emulsions (Figure 6) are raising high interest in the recent years. They are characterized by a long-term stability and have green connotations due to the absence of conventional emulsifiers. These attributes comply with the recent trends of food industry towards the use of sustainable and healthy technologies [16]. The stability of Pickering emulsions is related with the intrinsic properties of the oil and water phases (e.g., type, oil/water ratio, pH, ionic strength) and of the particle stabilizers (e.g., wettability, particle morphology, size and concentration). Particles presenting a contact angle (θ) below 90° are generally suitable for preparing O/W emulsions, whereas θ values greater than 90° indicate good stabilizers for W/O emulsions. At 90°, the particle is immersed equally in both phases [120].
Schematic representation of a Pickering emulsion putting in evidence the particle stabilizers where θ represent the contact angle.
Regarding natural-based particles, three main typologies of stabilizers can be used, namely nanoparticles, microgels and fibrils. Examples include protein derived stabilizers, namely nanoparticles based on corn zein, and colloidal particles of kafirin and gliadin [118, 121, 122, 123]. Although many polysaccharides have high hydrophilic character, some can include hydrophobic side groups (e.g., beet pectin and modified starch) or even active proteins attached to the surface (e.g., gum Arabic) [120], offering potential to act as Pickering stabilizers. Other polysaccharides widely used to produce Pickering bionanoparticles include chitin, chitosan and cellulose. To overcome particle’s limitations as Pickering stabilizers, the formation of complexes has been also proposed, namely complexes such as polysaccharide-polysaccharide, protein–protein, and polysaccharide-protein [124]. Examples include zein-xanthan [125], and tea water insoluble proteins/κ-carrageenan complexes [126].
In the context of the recent trends in Pickering emulsions, research aiming at finding new biological particles, the use of high internal phase emulsions (HIPPE), and the development of bio-based films from Pickering emulsions are becoming topics of high interest for the development of novel food applications. Table 4 presents an overview of recent works dealing with the preparation of Pickering emulsions based on novel biological particles together with the description of the main results envisaging potential food applications.
Particle materials | Main results | Reference |
---|---|---|
Apple pomace | Smaller particles led to emulsions with smaller droplet size, showing higher stability over time (30 days), in addition to improved physical properties (gel-like samples) and antioxidant activity. | [127] |
Bamboo shoots dietary fiber | The emulsions were stable for 4 weeks avoiding coalescence against pH and ionic strength changes and pasteurization conditions. | [128] |
Chitosan-sodium tripolyphosphate | The emulsions presented good thermal stability, showing potential to be applied as a food delivery system for essential oils. | [129] |
Gliadin-pectin | The emulsion with higher particles content (2%) showed suitable physical stability for 30 days, elastic-solid characteristics and good thermal stability (20–80°C). | [130] |
Hordein-chitosan | The emulsions exhibited good stability during storage (14 days, oil ratio = 0.5 and 0.6) and physical properties (elastic gel-like network). | [131] |
Pea protein | The emulsions with higher particle content showed stability against coalescence over 3 months. | [30] |
Sago starch nanocrystals | The emulsions were stable with no signs of creaming for over 2 months. | [132] |
Soy protein isolate-anthocyanin | The emulsions were reached a cream index of 17%, presenting stability for 7 weeks. They presented improved oxidative stability and resistance to | [133] |
Soy protein isolate-chitosan | Cream index values were very low, and the emulsion presented good stability to a broad range of ionic strength and mild temperature conditions (4–60°C). | [134] |
Tea protein | Emulsions with gel-like properties were produced, presenting no creaming over 50 days. | [135] |
Zein-corn fiber gum | High oil concentrations (oil ratio = 0.5) led to higher stability and the formation of a gel-like structure. | [136] |
Zein-gum arabic | The emulsion showed a high stability against coalescence and Ostwald ripening during 30 days of storage (oil ratio = 0.7). | [137] |
Zein-pectin | The emulsions maintained excellent physical stability for 1 month. In addition, they demonstrated good performance as delivery systems of essential oils. | [138] |
β-lactoglobulin-gum arabic | The particles provided stability against coalescence and Ostwald ripening for up to 12 weeks, in addition to improve chemical stability. | [139] |
Examples of bionanoparticles as Pickering stabilizers. All the systems are of O/W type.
HIPPEs are characterized by having a high volume fraction of internal phase (generally higher than 74%), together with relatively low particles concentration resulting in an extremely compacted droplet’s structure [140]. HIPPEs are becoming a novel approach of increasing interest in the food industry, since it combines diverse advantages, namely a semi-solid texture with the ability to encapsulate high amounts of bioactive compounds [141]. HIPPEs allow to control the droplet size distribution, manipulate the morphology and rheological properties, generally presenting enhanced stability against physical, chemical and microbiological stresses [142]. They are positioned as extremely promising substitutes for foods such as margarine, mayonnaise or ice creams [143, 144]. For example, Liu et al. studied wheat gluten as stabilizer in a HIPPE to develop a novel mayonnaise substitute [145]. They obtained excellent results concerning texture and sensory attributes when compared with commercial products.
Bio-based films made from hydrophilic particles added with hydrophobic compounds is another emerging approach in the scope of new applications developed from Pickering emulsions [146]. These strategies provide the ability to improve the stability of the base materials (hydrophilic), in addition to facilitate the combination with hydrophobic materials (e.g., waxes, fatty oils and oils) leading to systems with enhanced moisture barrier properties [147].
The wide variety of emulsion-based systems using natural emulsifiers makes their applicability attractive for various products, particularly in the food industry. The nature and function of emulsifiers, and the formed emulsion type (e.g., nano/micro-scale, simple or double character) can tailor appearance, sensorial characteristics, and attractiveness of foods. Among their diverse functions, the increasing use of emulsions as functionality carriers should be highlighted. In fact, recent works have demonstrated their potential and versatility for the encapsulation of flavors, and to protect and deliver specific bioactives in foods or beverages, helping to strengthen nutritional balances, and enabling the production of reduced-fat products. A summary of examples addressing new trends of emulsion-based products with potential in the food industry are included in Table 5, with some highlighs provided next.
Emulsion type | Oil phase | Emulsifier | Highlights | Reference |
---|---|---|---|---|
O/W | Vegetable oil | Orange pulp and peel powders | Base emulsions for food applications | [148] |
O/W | Orange essential oil | Pea protein concentrate and soy protein isolate | Encapsulation of flavors for the food industry (powder form) | [149] |
O/W | Sunflower, soybean, MCT, and orange oils | Crude saponins isolated from onion skin wastes | Food nanoemulsions (stable emulsions, except at acidic pH and high ionic strength) | [40] |
O/W | Paprika oleoresin | Whey protein and gum Arabic, and soy lecithin | Food and beverage systems for the delivery of carotenoids | [92] |
O/W | Hemp seed oil | Food nanoemulsions with enhanced nutritional properties | [150] | |
O/W | Almond, mustard, olive, and soyabean oils | Biosurfactants isolated from | Food emulsions with improved antibacterial capacity | [151] |
O/W | Corn and sunflower oils) / | Biosurfactant isolated from | Food emulsions with promising properties for salad dressings | [63] |
O1/W/O2 | O1 – Sunflower; O2 - palm oil | Primary emulsion - gelatin, xanthan gum; Secondary emulsion - solid fat crystals | Oil encapsulation systems for texturizing reduced-fat agents | [152] |
Applications of natural-based emulsifiers in food industry.
Lopes Francisco et al. [149] reported an emulsifying system with encapsulation potential based on commercial pea and soy proteins. The work involved the encapsulation of an orange essential oil rich in d-limonene using a O/W emulsion followed by spray drying to obtain powder microparticles. It was demonstrated the ability of pea and soy proteins to act as emulsifiers in the encapsulation of orange essential oil, getting a slightly higher efficiency if using soy protein as the natural emulsifier. These promising results can help consolidate a platform aiming at developing new protective systems to encapsulate flavors for foods, complying with the increasing demand from this industrial sector for natural-based systems.
At the nanoscale, Flores-Andrade et al. [92] reported the preparation of O/W nanoemulsions by high-pressure homogenization, using amphiphilic biopolymers to stabilize paprika oleoresin, namely whey protein, gum Arabic, phospholipids, and soy lecithin. The results demonstrated the effective oil encapsulation, preserving carotenoids (e.g., lipophilic colorants) from chemical degradation, besides positioning this strategy as an attractive route to design new protective and delivery carriers for bioactive compounds aimed at food and/or beverage products.
The potential of double emulsions was also demonstrated by Cetinkaya et al. [152] that evidenced the reduction of the saturated fat content in O1/W/O2 emulsions prepared by fat crystallization according to a two-stage process. Firstly, the primary O1/W emulsion was prepared using sunflower oil and xanthan gum and gelatin as emulsifiers, which was then stabilized in a second oil phase (palm oil), resulting in a structured O1/W/O2 system. Microstructure examination revealed that the accumulation of fat crystals at the interface contributed to stabilize the internal water phase containing the encapsulated sunflower phase. These complex structures showed potential to directly encapsulate hydrophobic oils and act as texturizing reduced-fat agents, which might be of particular interest for the edible oils industry.
This chapter presents an up-to-date overview of current trends in natural emulsifiers and their application in emulsion technology directed to food applications. For this purpose, first, the evolution of food emulsifiers’ scenario over the last 10 years was analyzed through the Bibliometrix-R package (RStudio) and VosViewer software. This analysis indicated a clear driving force towards using natural emulsifiers and the re-emerging importance of the Pickering emulsions. These facts are expected to impact the market growth following the prospectus of available market analysis. The six main identified families of natural emulsifiers were phospholipids, saponins, proteins, polysaccharides, emulsifiers from microbial sources and Pickering stabilizers. Some of them already find extensive use in practical food applications. However, others, mainly natural-based emulsifiers from microbial sources and Pickering stabilizers, despite their high potential, are still needing research investment and regulation clarification (e.g., related to the use of nanoparticles and the use of microbial strains classified as pathogenic in foods). From a technological perspective, the main concepts related to the typology, production methods, stabilization mechanisms, and instability phenomena were presented. Highlighting the increasing interest in Pickering emulsions, a summary of the most recent applications of these systems, including the so-called HIPPEs and their advantages in reduced-fat products development, was provided. To conclude, an analysis of current trends in food emulsion-based products was discussed, putting in evidence the emulsions increasing role as delivery systems of bioactives to support innovative fortified foods advances and the increasing interest in systems based on double emulsions, which provide the opportunity to combine bioactives of different nature. Overall, the field of natural-based emulsifiers combined with the new trends in emulsion technology can, hopefully, be the basis of a new generation of healthy and nutritious food products.
CIMO (UIDB/00690/2020) and AL LSRE-LCM (UIDB/50020/2020) funded by FCT/MCTES (PIDDAC). National funding by FCT, P.I., through the institutional scientific employment program-contract for Arantzazu Santamaria-Echart and Isabel P. Fernandes. FCT for the Research grants (SFRH/BD/148281/2019 of Samara C. da Silva, and SFRH/BD/147326/2019 of Stephany C. de Rezende). GreenHealth project (Norte-01-0145-FEDER-000042).
The authors declare no conflict of interest.
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On September, 29th 2006 he has won a post PhD fellowship from the university of Bologna (from October 2006 to October 2008), at the competitive examination he was ranked first in the industrial engineering area. He extensively served as referee for several international journals. He is author/coauthor of more than 100 research papers. He has been involved in some projects supported by MURST and European Community. 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Delac received his B.Sc.E.E. degree in 2003 and is currentlypursuing a Ph.D. degree at the University of Zagreb, Faculty of Electrical Engineering andComputing. His current research interests are digital image analysis, pattern recognition andbiometrics.",institutionString:null,institution:{name:"University of Zagreb",country:{name:"Croatia"}}},{id:"557",title:"Dr.",name:"Andon",middleName:"Venelinov",surname:"Topalov",slug:"andon-topalov",fullName:"Andon Topalov",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/557/images/1927_n.jpg",biography:"Dr. Andon V. Topalov received the MSc degree in Control Engineering from the Faculty of Information Systems, Technologies, and Automation at Moscow State University of Civil Engineering (MGGU) in 1979. He then received his PhD degree in Control Engineering from the Department of Automation and Remote Control at Moscow State Mining University (MGSU), Moscow, in 1984. 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Fortunately, pipeline inspection gauge (PIG) could accomplish the pipeline defect (corrosions, cracks, grooves, etc.) inspection effectively and meanwhile to localize these defects precisely by navigation sensors. The results are utilized for pipeline integrity management (PIM) and pipeline geographic information system construction. Generally, the urban underground pipeline presents with small-diameter and complicated-distribution properties, which are of great challenges for the pipeline defects positioning by PIG. This chapter focuses on in-depth research of the high-precision positioning method for small-diameter PIG navigation. In the beginning, the problems and system errors statement of MEMS SINS-based PIG are analyzed step by step. Then, the pipeline junction (PJ) identification method based on fast orthogonal search (FOS) is studied. After that, a PIG positioning system that comprises of micro-inertial/AGM/odometer/PJ is proposed, and also the application mechanism of extended Kalman filter and its smoothing technology on PIG navigation system is researched to improve the overall positioning precision for the small-diameter PIG. Finally, the proposed methods and research conclusions are verified by the indoor wheel robot simulation platform.",book:{id:"7386",slug:"advances-in-human-and-machine-navigation-systems",title:"Advances in Human and Machine Navigation Systems",fullTitle:"Advances in Human and Machine Navigation Systems"},signatures:"Lianwu Guan, Xu Xu, Yanbin Gao, Fanming Liu, Hanxiao Rong, Meng Wang and Aboelmagd Noureldin",authors:null},{id:"54800",doi:"10.5772/68121",title:"Human Action Recognition with RGB-D Sensors",slug:"human-action-recognition-with-rgb-d-sensors",totalDownloads:1713,totalCrossrefCites:3,totalDimensionsCites:5,abstract:"Human action recognition, also known as HAR, is at the foundation of many different applications related to behavioral analysis, surveillance, and safety, thus it has been a very active research area in the last years. 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The development of this technology in assessing open surgical skills is far behind when compared to minimally invasive surgery (MIS) training. Surgical skills such as suturing and tying surgical knots are assessed by an observational tool. It is labour-intensive and time-consuming. Therefore, we explored the potential use of motion tracking system as a non-observational assessment tool for basic surgical skills.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Shazrinizam Shaharan, Donncha M Ryan and Paul C Neary",authors:[{id:"37650",title:"Prof.",name:"Paul",middleName:null,surname:"Neary",slug:"paul-neary",fullName:"Paul Neary"},{id:"153270",title:"Mr.",name:"Donncha",middleName:null,surname:"Ryan",slug:"donncha-ryan",fullName:"Donncha Ryan"},{id:"196439",title:"Dr.",name:"Shazrinizam",middleName:null,surname:"Shaharan",slug:"shazrinizam-shaharan",fullName:"Shazrinizam Shaharan"}]},{id:"55073",doi:"10.5772/68119",title:"Gait Recognition",slug:"gait-recognition",totalDownloads:2371,totalCrossrefCites:4,totalDimensionsCites:4,abstract:"Gait recognition has received increasing attention as a remote biometric identification technology, i.e. it can achieve identification at the long distance that few other identification technologies can work. It shows enormous potential to apply in the field of criminal investigation, medical treatment, identity recognition, human‐computer interaction and so on. In this chapter, we introduce the state‐of‐the‐art gait recognition techniques, which include 3D‐based and 2D‐based methods, in the first part. And considering the advantages of 3D‐based methods, their related datasets are introduced as well as our gait database with both 2D silhouette images and 3D joints information in the second part. Given our gait dataset, a human walking model and the corresponding static and dynamic feature extraction are presented, which are verified to be view‐invariant, in the third part. And some gait‐based applications are introduced.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Jiande Sun, Yufei Wang and Jing Li",authors:[{id:"197788",title:"Prof.",name:"Jiande",middleName:null,surname:"Sun",slug:"jiande-sun",fullName:"Jiande Sun"},{id:"197789",title:"M.Sc.",name:"Yufei",middleName:null,surname:"Wang",slug:"yufei-wang",fullName:"Yufei Wang"},{id:"197790",title:"Ms.",name:"Jing",middleName:null,surname:"Li",slug:"jing-li",fullName:"Jing Li"}]},{id:"63770",doi:"10.5772/intechopen.79886",title:"Vision-Based Tactile Paving Detection Method in Navigation Systems for Visually Impaired Persons",slug:"vision-based-tactile-paving-detection-method-in-navigation-systems-for-visually-impaired-persons",totalDownloads:1056,totalCrossrefCites:0,totalDimensionsCites:2,abstract:"In general, a visually impaired person relies on guide canes in order to walk outside besides depending only on a tactile pavement as a warning and directional tool in order to avoid any obstructions or hazardous situations. However, still a lot of training is needed in order to recognize the tactile pattern, and it is quite difficult for persons who have recently become visually impaired. This chapter describes the development and evaluation of vision-based tactile paving detection method for visually impaired persons. Some experiments will be conducted on how it works to detect the tactile pavement and identify the shape of tactile pattern. In this experiment, a vision-based method is proposed by using MATLAB including the Arduino platform and speaker as guidance tools. The output of this system based on the result found from tactile detection in MATLAB then produces auditory output and notifies the visually impaired about the type of tactile detected. Consequently, the development of tactile pavement detection system can be used by visually impaired persons for easy detection and navigation purposes.",book:{id:"7386",slug:"advances-in-human-and-machine-navigation-systems",title:"Advances in Human and Machine Navigation Systems",fullTitle:"Advances in Human and Machine Navigation Systems"},signatures:"Anuar Bin Mohamed Kassim, Takashi Yasuno, Hiroshi Suzuki, Mohd Shahrieel Mohd Aras, Ahmad Zaki Shukor, Hazriq Izzuan Jaafar and Fairul Azni Jafar",authors:null}],mostDownloadedChaptersLast30Days:[{id:"67001",title:"Optimization of NOE Flights Sensors and Their Integration",slug:"optimization-of-noe-flights-sensors-and-their-integration",totalDownloads:1246,totalCrossrefCites:0,totalDimensionsCites:0,abstract:"This chapter unveils an enhancement strategy for nap-of-the-earth. The nap-of-the-earth (NOE) mode is the most energizing, most unsafe, and is generally the slowest. Military aircraft to maintain a strategic distance from opponent detection and assault in a high-thread circumstance use it. NOE used to limit discovery by the ground-based radar, targets and the control system. The radar altimeter (RA) or terrain following radar (TFR), terrain awareness and warning system (TAWS) used to identify the curbs during flying in NOE flights. Here, while the plane is at the nap of the earth activity, the speed and the height must be moderate as effectively decided. The terrain following radar (TFR) keeps up the altitude from the beginning. Therefore, we analyze the issue to expand the performance of the airplane by extending the terrain by a few modes of the TAWS, which given by various aviation authorities1. Further to this, different TAWS modes of action, explanation of mode selection and progression in TAWS clarified in detail. This chapter displays the MATLAB programme for a few patterns of TAWS mission, and simulation of the flight path for the excessive terrain closure rate from mode two operation of the flight.",book:{id:"7386",slug:"advances-in-human-and-machine-navigation-systems",title:"Advances in Human and Machine Navigation Systems",fullTitle:"Advances in Human and Machine Navigation Systems"},signatures:"Tamilselvam Nallusamy and Prasanalakshmi Balaji",authors:null},{id:"55073",title:"Gait Recognition",slug:"gait-recognition",totalDownloads:2370,totalCrossrefCites:4,totalDimensionsCites:4,abstract:"Gait recognition has received increasing attention as a remote biometric identification technology, i.e. it can achieve identification at the long distance that few other identification technologies can work. It shows enormous potential to apply in the field of criminal investigation, medical treatment, identity recognition, human‐computer interaction and so on. 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And some gait‐based applications are introduced.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Jiande Sun, Yufei Wang and Jing Li",authors:[{id:"197788",title:"Prof.",name:"Jiande",middleName:null,surname:"Sun",slug:"jiande-sun",fullName:"Jiande Sun"},{id:"197789",title:"M.Sc.",name:"Yufei",middleName:null,surname:"Wang",slug:"yufei-wang",fullName:"Yufei Wang"},{id:"197790",title:"Ms.",name:"Jing",middleName:null,surname:"Li",slug:"jing-li",fullName:"Jing Li"}]},{id:"54800",title:"Human Action Recognition with RGB-D Sensors",slug:"human-action-recognition-with-rgb-d-sensors",totalDownloads:1712,totalCrossrefCites:3,totalDimensionsCites:5,abstract:"Human action recognition, also known as HAR, is at the foundation of many different applications related to behavioral analysis, surveillance, and safety, thus it has been a very active research area in the last years. The release of inexpensive RGB-D sensors fostered researchers working in this field because depth data simplify the processing of visual data that could be otherwise difficult using classic RGB devices. Furthermore, the availability of depth data allows to implement solutions that are unobtrusive and privacy preserving with respect to classic video-based analysis. In this scenario, the aim of this chapter is to review the most salient techniques for HAR based on depth signal processing, providing some details on a specific method based on temporal pyramid of key poses, evaluated on the well-known MSR Action3D dataset.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Enea Cippitelli, Ennio Gambi and Susanna Spinsante",authors:[{id:"197890",title:"Ph.D. Student",name:"Enea",middleName:null,surname:"Cippitelli",slug:"enea-cippitelli",fullName:"Enea Cippitelli"},{id:"198036",title:"Prof.",name:"Ennio",middleName:null,surname:"Gambi",slug:"ennio-gambi",fullName:"Ennio Gambi"},{id:"198037",title:"Dr.",name:"Susanna",middleName:null,surname:"Spinsante",slug:"susanna-spinsante",fullName:"Susanna Spinsante"}]},{id:"54897",title:"Audio‐Visual Speaker Tracking",slug:"audio-visual-speaker-tracking",totalDownloads:1473,totalCrossrefCites:3,totalDimensionsCites:2,abstract:"Target motion tracking found its application in interdisciplinary fields, including but not limited to surveillance and security, forensic science, intelligent transportation system, driving assistance, monitoring prohibited area, medical science, robotics, action and expression recognition, individual speaker discrimination in multi‐speaker environments and video conferencing in the fields of computer vision and signal processing. Among these applications, speaker tracking in enclosed spaces has been gaining relevance due to the widespread advances of devices and technologies and the necessity for seamless solutions in real‐time tracking and localization of speakers. However, speaker tracking is a challenging task in real‐life scenarios as several distinctive issues influence the tracking process, such as occlusions and an unknown number of speakers. One approach to overcome these issues is to use multi‐modal information, as it conveys complementary information about the state of the speakers compared to single‐modal tracking. To use multi‐modal information, several approaches have been proposed which can be classified into two categories, namely deterministic and stochastic. This chapter aims at providing multimedia researchers with a state‐of‐the‐art overview of tracking methods, which are used for combining multiple modalities to accomplish various multimedia analysis tasks, classifying them into different categories and listing new and future trends in this field.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Volkan Kılıç and Wenwu Wang",authors:[{id:"149571",title:"Dr.",name:"Wenwu",middleName:null,surname:"Wang",slug:"wenwu-wang",fullName:"Wenwu Wang"},{id:"197235",title:"Dr.",name:"Volkan",middleName:null,surname:"Kılıç",slug:"volkan-kilic",fullName:"Volkan Kılıç"}]},{id:"55646",title:"Motion Tracking System in Surgical Training",slug:"motion-tracking-system-in-surgical-training",totalDownloads:1957,totalCrossrefCites:1,totalDimensionsCites:4,abstract:"Introduction: Simulation technology is evolving and becoming the focus of attention in surgical training. The development of this technology in assessing open surgical skills is far behind when compared to minimally invasive surgery (MIS) training. Surgical skills such as suturing and tying surgical knots are assessed by an observational tool. It is labour-intensive and time-consuming. Therefore, we explored the potential use of motion tracking system as a non-observational assessment tool for basic surgical skills.",book:{id:"5783",slug:"motion-tracking-and-gesture-recognition",title:"Motion Tracking and Gesture Recognition",fullTitle:"Motion Tracking and Gesture Recognition"},signatures:"Shazrinizam Shaharan, Donncha M Ryan and Paul C Neary",authors:[{id:"37650",title:"Prof.",name:"Paul",middleName:null,surname:"Neary",slug:"paul-neary",fullName:"Paul Neary"},{id:"153270",title:"Mr.",name:"Donncha",middleName:null,surname:"Ryan",slug:"donncha-ryan",fullName:"Donncha Ryan"},{id:"196439",title:"Dr.",name:"Shazrinizam",middleName:null,surname:"Shaharan",slug:"shazrinizam-shaharan",fullName:"Shazrinizam Shaharan"}]}],onlineFirstChaptersFilter:{topicId:"260",limit:6,offset:0},onlineFirstChaptersCollection:[],onlineFirstChaptersTotal:0},preDownload:{success:null,errors:{}},subscriptionForm:{success:null,errors:{}},aboutIntechopen:{},privacyPolicy:{},peerReviewing:{},howOpenAccessPublishingWithIntechopenWorks:{},sponsorshipBooks:{sponsorshipBooks:[],offset:0,limit:8,total:null},allSeries:{pteSeriesList:[{id:"14",title:"Artificial Intelligence",numberOfPublishedBooks:9,numberOfPublishedChapters:90,numberOfOpenTopics:6,numberOfUpcomingTopics:0,issn:"2633-1403",doi:"10.5772/intechopen.79920",isOpenForSubmission:!0},{id:"7",title:"Biomedical Engineering",numberOfPublishedBooks:12,numberOfPublishedChapters:108,numberOfOpenTopics:3,numberOfUpcomingTopics:0,issn:"2631-5343",doi:"10.5772/intechopen.71985",isOpenForSubmission:!0}],lsSeriesList:[{id:"11",title:"Biochemistry",numberOfPublishedBooks:33,numberOfPublishedChapters:330,numberOfOpenTopics:4,numberOfUpcomingTopics:0,issn:"2632-0983",doi:"10.5772/intechopen.72877",isOpenForSubmission:!0},{id:"25",title:"Environmental Sciences",numberOfPublishedBooks:1,numberOfPublishedChapters:19,numberOfOpenTopics:4,numberOfUpcomingTopics:0,issn:"2754-6713",doi:"10.5772/intechopen.100362",isOpenForSubmission:!0},{id:"10",title:"Physiology",numberOfPublishedBooks:14,numberOfPublishedChapters:145,numberOfOpenTopics:4,numberOfUpcomingTopics:0,issn:"2631-8261",doi:"10.5772/intechopen.72796",isOpenForSubmission:!0}],hsSeriesList:[{id:"3",title:"Dentistry",numberOfPublishedBooks:9,numberOfPublishedChapters:141,numberOfOpenTopics:2,numberOfUpcomingTopics:0,issn:"2631-6218",doi:"10.5772/intechopen.71199",isOpenForSubmission:!0},{id:"6",title:"Infectious Diseases",numberOfPublishedBooks:13,numberOfPublishedChapters:123,numberOfOpenTopics:4,numberOfUpcomingTopics:0,issn:"2631-6188",doi:"10.5772/intechopen.71852",isOpenForSubmission:!0},{id:"13",title:"Veterinary Medicine and Science",numberOfPublishedBooks:11,numberOfPublishedChapters:112,numberOfOpenTopics:3,numberOfUpcomingTopics:0,issn:"2632-0517",doi:"10.5772/intechopen.73681",isOpenForSubmission:!0}],sshSeriesList:[{id:"22",title:"Business, Management and Economics",numberOfPublishedBooks:1,numberOfPublishedChapters:22,numberOfOpenTopics:3,numberOfUpcomingTopics:0,issn:"2753-894X",doi:"10.5772/intechopen.100359",isOpenForSubmission:!0},{id:"23",title:"Education and Human Development",numberOfPublishedBooks:0,numberOfPublishedChapters:11,numberOfOpenTopics:1,numberOfUpcomingTopics:1,issn:null,doi:"10.5772/intechopen.100360",isOpenForSubmission:!0},{id:"24",title:"Sustainable Development",numberOfPublishedBooks:1,numberOfPublishedChapters:19,numberOfOpenTopics:5,numberOfUpcomingTopics:0,issn:"2753-6580",doi:"10.5772/intechopen.100361",isOpenForSubmission:!0}],testimonialsList:[{id:"6",text:"It is great to work with the IntechOpen to produce a worthwhile collection of research that also becomes a great educational resource and guide for future research endeavors.",author:{id:"259298",name:"Edward",surname:"Narayan",institutionString:null,profilePictureURL:"https://mts.intechopen.com/storage/users/259298/images/system/259298.jpeg",slug:"edward-narayan",institution:{id:"3",name:"University of Queensland",country:{id:null,name:"Australia"}}}},{id:"13",text:"The collaboration with and support of the technical staff of IntechOpen is fantastic. The whole process of submitting an article and editing of the submitted article goes extremely smooth and fast, the number of reads and downloads of chapters is high, and the contributions are also frequently cited.",author:{id:"55578",name:"Antonio",surname:"Jurado-Navas",institutionString:null,profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0030O00002bRisIQAS/Profile_Picture_1626166543950",slug:"antonio-jurado-navas",institution:{id:"720",name:"University of Malaga",country:{id:null,name:"Spain"}}}}]},series:{item:{id:"11",title:"Biochemistry",doi:"10.5772/intechopen.72877",issn:"2632-0983",scope:"Biochemistry, the study of chemical transformations occurring within living organisms, impacts all areas of life sciences, from molecular crystallography and genetics to ecology, medicine, and population biology. Biochemistry examines macromolecules - proteins, nucleic acids, carbohydrates, and lipids – and their building blocks, structures, functions, and interactions. Much of biochemistry is devoted to enzymes, proteins that catalyze chemical reactions, enzyme structures, mechanisms of action and their roles within cells. Biochemistry also studies small signaling molecules, coenzymes, inhibitors, vitamins, and hormones, which play roles in life processes. Biochemical experimentation, besides coopting classical chemistry methods, e.g., chromatography, adopted new techniques, e.g., X-ray diffraction, electron microscopy, NMR, radioisotopes, and developed sophisticated microbial genetic tools, e.g., auxotroph mutants and their revertants, fermentation, etc. More recently, biochemistry embraced the ‘big data’ omics systems. Initial biochemical studies have been exclusively analytic: dissecting, purifying, and examining individual components of a biological system; in the apt words of Efraim Racker (1913 –1991), “Don’t waste clean thinking on dirty enzymes.” Today, however, biochemistry is becoming more agglomerative and comprehensive, setting out to integrate and describe entirely particular biological systems. The ‘big data’ metabolomics can define the complement of small molecules, e.g., in a soil or biofilm sample; proteomics can distinguish all the comprising proteins, e.g., serum; metagenomics can identify all the genes in a complex environment, e.g., the bovine rumen. This Biochemistry Series will address the current research on biomolecules and the emerging trends with great promise.",coverUrl:"https://cdn.intechopen.com/series/covers/11.jpg",latestPublicationDate:"August 2nd, 2022",hasOnlineFirst:!0,numberOfPublishedBooks:33,editor:{id:"31610",title:"Dr.",name:"Miroslav",middleName:null,surname:"Blumenberg",slug:"miroslav-blumenberg",fullName:"Miroslav Blumenberg",profilePictureURL:"https://mts.intechopen.com/storage/users/31610/images/system/31610.jpg",biography:"Miroslav Blumenberg, Ph.D., was born in Subotica and received his BSc in Belgrade, Yugoslavia. He completed his Ph.D. at MIT in Organic Chemistry; he followed up his Ph.D. with two postdoctoral study periods at Stanford University. Since 1983, he has been a faculty member of the RO Perelman Department of Dermatology, NYU School of Medicine, where he is codirector of a training grant in cutaneous biology. Dr. Blumenberg’s research is focused on the epidermis, expression of keratin genes, transcription profiling, keratinocyte differentiation, inflammatory diseases and cancers, and most recently the effects of the microbiome on the skin. He has published more than 100 peer-reviewed research articles and graduated numerous Ph.D. and postdoctoral students.",institutionString:null,institution:{name:"New York University Langone Medical Center",institutionURL:null,country:{name:"United States of America"}}},editorTwo:null,editorThree:null},subseries:{paginationCount:4,paginationItems:[{id:"14",title:"Cell and Molecular Biology",coverUrl:"https://cdn.intechopen.com/series_topics/covers/14.jpg",isOpenForSubmission:!0,editor:{id:"165627",title:"Dr.",name:"Rosa María",middleName:null,surname:"Martínez-Espinosa",slug:"rosa-maria-martinez-espinosa",fullName:"Rosa María Martínez-Espinosa",profilePictureURL:"https://mts.intechopen.com/storage/users/165627/images/system/165627.jpeg",biography:"Dr. Rosa María Martínez-Espinosa has been a Spanish Full Professor since 2020 (Biochemistry and Molecular Biology) and is currently Vice-President of International Relations and Cooperation development and leader of the research group 'Applied Biochemistry” (University of Alicante, Spain). Other positions she has held at the university include Vice-Dean of Master Programs, Vice-Dean of the Degree in Biology and Vice-Dean for Mobility and Enterprise and Engagement at the Faculty of Science (University of Alicante). She received her Bachelor in Biology in 1998 (University of Alicante) and her PhD in 2003 (Biochemistry, University of Alicante). She undertook post-doctoral research at the University of East Anglia (Norwich, U.K. 2004-2005; 2007-2008).\nHer multidisciplinary research focuses on investigating archaea and their potential applications in biotechnology. She has an H-index of 21. She has authored one patent and has published more than 70 indexed papers and around 60 book chapters.\nShe has contributed to more than 150 national and international meetings during the last 15 years. Her research interests include archaea metabolism, enzymes purification and characterization, gene regulation, carotenoids and bioplastics production, antioxidant\ncompounds, waste water treatments, and brines bioremediation.\nRosa María’s other roles include editorial board member for several journals related\nto biochemistry, reviewer for more than 60 journals (biochemistry, molecular biology, biotechnology, chemistry and microbiology) and president of several organizing committees in international meetings related to the N-cycle or respiratory processes.",institutionString:null,institution:{name:"University of Alicante",institutionURL:null,country:{name:"Spain"}}},editorTwo:null,editorThree:null},{id:"15",title:"Chemical Biology",coverUrl:"https://cdn.intechopen.com/series_topics/covers/15.jpg",isOpenForSubmission:!0,editor:{id:"441442",title:"Dr.",name:"Şükrü",middleName:null,surname:"Beydemir",slug:"sukru-beydemir",fullName:"Şükrü Beydemir",profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0033Y00003GsUoIQAV/Profile_Picture_1634557147521",biography:"Dr. Şükrü Beydemir obtained a BSc in Chemistry in 1995 from Yüzüncü Yıl University, MSc in Biochemistry in 1998, and PhD in Biochemistry in 2002 from Atatürk University, Turkey. He performed post-doctoral studies at Max-Planck Institute, Germany, and University of Florence, Italy in addition to making several scientific visits abroad. He currently works as a Full Professor of Biochemistry in the Faculty of Pharmacy, Anadolu University, Turkey. Dr. Beydemir has published over a hundred scientific papers spanning protein biochemistry, enzymology and medicinal chemistry, reviews, book chapters and presented several conferences to scientists worldwide. He has received numerous publication awards from various international scientific councils. He serves in the Editorial Board of several international journals. Dr. Beydemir is also Rector of Bilecik Şeyh Edebali University, Turkey.",institutionString:null,institution:{name:"Anadolu University",institutionURL:null,country:{name:"Turkey"}}},editorTwo:{id:"13652",title:"Prof.",name:"Deniz",middleName:null,surname:"Ekinci",slug:"deniz-ekinci",fullName:"Deniz Ekinci",profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0030O00002aYLT1QAO/Profile_Picture_1634557223079",biography:"Dr. Deniz Ekinci obtained a BSc in Chemistry in 2004, MSc in Biochemistry in 2006, and PhD in Biochemistry in 2009 from Atatürk University, Turkey. He studied at Stetson University, USA, in 2007-2008 and at the Max Planck Institute of Molecular Cell Biology and Genetics, Germany, in 2009-2010. Dr. Ekinci currently works as a Full Professor of Biochemistry in the Faculty of Agriculture and is the Head of the Enzyme and Microbial Biotechnology Division, Ondokuz Mayıs University, Turkey. He is a member of the Turkish Biochemical Society, American Chemical Society, and German Genetics society. Dr. Ekinci published around ninety scientific papers, reviews and book chapters, and presented several conferences to scientists. He has received numerous publication awards from several scientific councils. Dr. Ekinci serves as the Editor in Chief of four international books and is involved in the Editorial Board of several international journals.",institutionString:null,institution:{name:"Ondokuz Mayıs University",institutionURL:null,country:{name:"Turkey"}}},editorThree:null},{id:"17",title:"Metabolism",coverUrl:"https://cdn.intechopen.com/series_topics/covers/17.jpg",isOpenForSubmission:!0,editor:{id:"138626",title:"Dr.",name:"Yannis",middleName:null,surname:"Karamanos",slug:"yannis-karamanos",fullName:"Yannis Karamanos",profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0030O00002g6Jv2QAE/Profile_Picture_1629356660984",biography:"Yannis Karamanos, born in Greece in 1953, completed his pre-graduate studies at the Université Pierre et Marie Curie, Paris, then his Masters and Doctoral degree at the Université de Lille (1983). He was associate professor at the University of Limoges (1987) before becoming full professor of biochemistry at the Université d’Artois (1996). He worked on the structure-function relationships of glycoconjugates and his main project was the investigations on the biological roles of the de-N-glycosylation enzymes (Endo-N-acetyl-β-D-glucosaminidase and peptide-N4-(N-acetyl-β-glucosaminyl) asparagine amidase). From 2002 he contributes to the understanding of the Blood-brain barrier functioning using proteomics approaches. He has published more than 70 papers. His teaching areas are energy metabolism and regulation, integration and organ specialization and metabolic adaptation.",institutionString:null,institution:{name:"Artois University",institutionURL:null,country:{name:"France"}}},editorTwo:null,editorThree:null},{id:"18",title:"Proteomics",coverUrl:"https://cdn.intechopen.com/series_topics/covers/18.jpg",isOpenForSubmission:!0,editor:{id:"200689",title:"Prof.",name:"Paolo",middleName:null,surname:"Iadarola",slug:"paolo-iadarola",fullName:"Paolo Iadarola",profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0030O00002bSCl8QAG/Profile_Picture_1623568118342",biography:"Paolo Iadarola graduated with a degree in Chemistry from the University of Pavia (Italy) in July 1972. He then worked as an Assistant Professor at the Faculty of Science of the same University until 1984. In 1985, Prof. Iadarola became Associate Professor at the Department of Biology and Biotechnologies of the University of Pavia and retired in October 2017. Since then, he has been working as an Adjunct Professor in the same Department at the University of Pavia. His research activity during the first years was primarily focused on the purification and structural characterization of enzymes from animal and plant sources. During this period, Prof. Iadarola familiarized himself with the conventional techniques used in column chromatography, spectrophotometry, manual Edman degradation, and electrophoresis). Since 1995, he has been working on: i) the determination in biological fluids (serum, urine, bronchoalveolar lavage, sputum) of proteolytic activities involved in the degradation processes of connective tissue matrix, and ii) on the identification of biological markers of lung diseases. In this context, he has developed and validated new methodologies (e.g., Capillary Electrophoresis coupled to Laser-Induced Fluorescence, CE-LIF) whose application enabled him to determine both the amounts of biochemical markers (Desmosines) in urine/serum of patients affected by Chronic Obstructive Pulmonary Disease (COPD) and the activity of proteolytic enzymes (Human Neutrophil Elastase, Cathepsin G, Pseudomonas aeruginosa elastase) in sputa of these patients. More recently, Prof. Iadarola was involved in developing techniques such as two-dimensional electrophoresis coupled to liquid chromatography/mass spectrometry (2DE-LC/MS) for the proteomic analysis of biological fluids aimed at the identification of potential biomarkers of different lung diseases. He is the author of about 150 publications (According to Scopus: H-Index: 23; Total citations: 1568- According to WOS: H-Index: 20; Total Citations: 1296) of peer-reviewed international journals. He is a Consultant Reviewer for several journals, including the Journal of Chromatography A, Journal of Chromatography B, Plos ONE, Proteomes, International Journal of Molecular Science, Biotech, Electrophoresis, and others. He is also Associate Editor of Biotech.",institutionString:null,institution:{name:"University of Pavia",institutionURL:null,country:{name:"Italy"}}},editorTwo:{id:"201414",title:"Dr.",name:"Simona",middleName:null,surname:"Viglio",slug:"simona-viglio",fullName:"Simona Viglio",profilePictureURL:"https://s3.us-east-1.amazonaws.com/intech-files/0030O00002bRKDHQA4/Profile_Picture_1630402531487",biography:"Simona Viglio is an Associate Professor of Biochemistry at the Department of Molecular Medicine at the University of Pavia. She has been working since 1995 on the determination of proteolytic enzymes involved in the degradation process of connective tissue matrix and on the identification of biological markers of lung diseases. She gained considerable experience in developing and validating new methodologies whose applications allowed her to determine both the amount of biomarkers (Desmosine and Isodesmosine) in the urine of patients affected by COPD, and the activity of proteolytic enzymes (HNE, Cathepsin G, Pseudomonas aeruginosa elastase) in the sputa of these patients. Simona Viglio was also involved in research dealing with the supplementation of amino acids in patients with brain injury and chronic heart failure. She is presently engaged in the development of 2-DE and LC-MS techniques for the study of proteomics in biological fluids. The aim of this research is the identification of potential biomarkers of lung diseases. She is an author of about 90 publications (According to Scopus: H-Index: 23; According to WOS: H-Index: 20) on peer-reviewed journals, a member of the “Società Italiana di Biochimica e Biologia Molecolare,“ and a Consultant Reviewer for International Journal of Molecular Science, Journal of Chromatography A, COPD, Plos ONE and Nutritional Neuroscience.",institutionString:null,institution:{name:"University of Pavia",institutionURL:null,country:{name:"Italy"}}},editorThree:null}]},overviewPageOFChapters:{paginationCount:42,paginationItems:[{id:"82914",title:"Glance on the Critical Role of IL-23 Receptor Gene Variations in Inflammation-Induced Carcinogenesis",doi:"10.5772/intechopen.105049",signatures:"Mohammed El-Gedamy",slug:"glance-on-the-critical-role-of-il-23-receptor-gene-variations-in-inflammation-induced-carcinogenesis",totalDownloads:15,totalCrossrefCites:0,totalDimensionsCites:0,authors:null,book:{title:"Chemokines Updates",coverURL:"https://cdn.intechopen.com/books/images_new/11672.jpg",subseries:{id:"18",title:"Proteomics"}}},{id:"82875",title:"Lipidomics as a Tool in the Diagnosis and Clinical Therapy",doi:"10.5772/intechopen.105857",signatures:"María Elizbeth Alvarez Sánchez, Erick Nolasco Ontiveros, Rodrigo Arreola, Adriana Montserrat Espinosa González, Ana María García Bores, Roberto Eduardo López Urrutia, Ignacio Peñalosa Castro, María del Socorro Sánchez Correa and Edgar Antonio Estrella Parra",slug:"lipidomics-as-a-tool-in-the-diagnosis-and-clinical-therapy",totalDownloads:7,totalCrossrefCites:0,totalDimensionsCites:0,authors:null,book:{title:"Fatty Acids - Recent Advances",coverURL:"https://cdn.intechopen.com/books/images_new/11669.jpg",subseries:{id:"17",title:"Metabolism"}}},{id:"82440",title:"Lipid Metabolism and Associated Molecular Signaling Events in Autoimmune Disease",doi:"10.5772/intechopen.105746",signatures:"Mohan Vanditha, Sonu Das and Mathew John",slug:"lipid-metabolism-and-associated-molecular-signaling-events-in-autoimmune-disease",totalDownloads:17,totalCrossrefCites:0,totalDimensionsCites:0,authors:null,book:{title:"Fatty Acids - Recent Advances",coverURL:"https://cdn.intechopen.com/books/images_new/11669.jpg",subseries:{id:"17",title:"Metabolism"}}},{id:"82483",title:"Oxidative Stress in Cardiovascular Diseases",doi:"10.5772/intechopen.105891",signatures:"Laura Mourino-Alvarez, Tamara Sastre-Oliva, Nerea Corbacho-Alonso and Maria G. Barderas",slug:"oxidative-stress-in-cardiovascular-diseases",totalDownloads:10,totalCrossrefCites:0,totalDimensionsCites:0,authors:null,book:{title:"Importance of Oxidative Stress and Antioxidant System in Health and Disease",coverURL:"https://cdn.intechopen.com/books/images_new/11671.jpg",subseries:{id:"15",title:"Chemical Biology"}}}]},overviewPagePublishedBooks:{paginationCount:33,paginationItems:[{type:"book",id:"7006",title:"Biochemistry and Health Benefits of Fatty Acids",subtitle:null,coverURL:"https://cdn.intechopen.com/books/images_new/7006.jpg",slug:"biochemistry-and-health-benefits-of-fatty-acids",publishedDate:"December 19th 2018",editedByType:"Edited by",bookSignature:"Viduranga Waisundara",hash:"c93a00abd68b5eba67e5e719f67fd20b",volumeInSeries:1,fullTitle:"Biochemistry and Health Benefits of Fatty Acids",editors:[{id:"194281",title:"Dr.",name:"Viduranga Y.",middleName:null,surname:"Waisundara",slug:"viduranga-y.-waisundara",fullName:"Viduranga Y. Waisundara",profilePictureURL:"https://mts.intechopen.com/storage/users/194281/images/system/194281.jpg",biography:"Dr. Viduranga Waisundara obtained her Ph.D. in Food Science\nand Technology from the Department of Chemistry, National\nUniversity of Singapore, in 2010. She was a lecturer at Temasek Polytechnic, Singapore from July 2009 to March 2013.\nShe relocated to her motherland of Sri Lanka and spearheaded the Functional Food Product Development Project at the\nNational Institute of Fundamental Studies from April 2013 to\nOctober 2016. She was a senior lecturer on a temporary basis at the Department of\nFood Technology, Faculty of Technology, Rajarata University of Sri Lanka. She is\ncurrently Deputy Principal of the Australian College of Business and Technology –\nKandy Campus, Sri Lanka. She is also the Global Harmonization Initiative (GHI)",institutionString:"Australian College of Business & Technology",institution:{name:"Kobe College",institutionURL:null,country:{name:"Japan"}}}]},{type:"book",id:"6820",title:"Keratin",subtitle:null,coverURL:"https://cdn.intechopen.com/books/images_new/6820.jpg",slug:"keratin",publishedDate:"December 19th 2018",editedByType:"Edited by",bookSignature:"Miroslav Blumenberg",hash:"6def75cd4b6b5324a02b6dc0359896d0",volumeInSeries:2,fullTitle:"Keratin",editors:[{id:"31610",title:"Dr.",name:"Miroslav",middleName:null,surname:"Blumenberg",slug:"miroslav-blumenberg",fullName:"Miroslav Blumenberg",profilePictureURL:"https://mts.intechopen.com/storage/users/31610/images/system/31610.jpg",biography:"Miroslav Blumenberg, Ph.D., was born in Subotica and received his BSc in Belgrade, Yugoslavia. He completed his Ph.D. at MIT in Organic Chemistry; he followed up his Ph.D. with two postdoctoral study periods at Stanford University. Since 1983, he has been a faculty member of the RO Perelman Department of Dermatology, NYU School of Medicine, where he is codirector of a training grant in cutaneous biology. Dr. Blumenberg’s research is focused on the epidermis, expression of keratin genes, transcription profiling, keratinocyte differentiation, inflammatory diseases and cancers, and most recently the effects of the microbiome on the skin. He has published more than 100 peer-reviewed research articles and graduated numerous Ph.D. and postdoctoral students.",institutionString:null,institution:{name:"New York University Langone Medical Center",institutionURL:null,country:{name:"United States of America"}}}]},{type:"book",id:"7978",title:"Vitamin A",subtitle:null,coverURL:"https://cdn.intechopen.com/books/images_new/7978.jpg",slug:"vitamin-a",publishedDate:"May 15th 2019",editedByType:"Edited by",bookSignature:"Leila Queiroz Zepka, Veridiana Vera de Rosso and Eduardo Jacob-Lopes",hash:"dad04a658ab9e3d851d23705980a688b",volumeInSeries:3,fullTitle:"Vitamin A",editors:[{id:"261969",title:"Dr.",name:"Leila",middleName:null,surname:"Queiroz Zepka",slug:"leila-queiroz-zepka",fullName:"Leila Queiroz Zepka",profilePictureURL:"https://mts.intechopen.com/storage/users/261969/images/system/261969.png",biography:"Prof. Dr. Leila Queiroz Zepka is currently an associate professor in the Department of Food Technology and Science, Federal University of Santa Maria, Brazil. 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